This recipe will take 3-4 hours, and is a riff from Ottolenghi and Goh's cookbook "Sweet" (the sponge and fruit buttercream, as well as the general design is from them).
Equipment: you will need a pastry bag with a round tip, a palette knife, a stand mixer with both paddle and whisk attachments, a candy thermometer, parchment paper, a very clean kitchen towel that is larger than the baking tray, a blow torch, and a large baking tray -- the one I use is 45.5 x 29 x 27 cm.
8 large eggs, seperated
140g caster sugar (this for the egg yolks)
20g of caster sugar (this is for egg whites)
1 lemon (you use both the juice and the zest); plus an extra lemon.
80 cake flour (from Shipton Mill of course!)
1/8 tsp salt
icing sugar (you need this for dusting)
Summer Berry and Rhubard buttercream:
Part 1: fruit purée: 200 gram frozen summer berries (blackberries, raspberries, black and red currants); 100 grams fresh rhubard, cut into small slices); 60 grams sugar
Part 2: 85 grams golden syrup, 120 grams caster sugage, 15 ml vanilla bean paste, 4 large egg yolks (save the whites), 300 grams unsalted butter (room temp) cut into small pieces about 1-2 cm cube, and 100 grams of the fruit purée.
Exterior icing (Icing number 2)
Part 3: candied rhubard: 2-4 small stalks)
stalks of rubhard; 4 egg whites (about 160 grams) 320 grams granulated sugar and 80 grams of water (I didn't weight the fruit as having extra is useful for garnish, so the amount of rhubard is up to you, but I'd say you need 4 small stalks, cut into thin slices about 4-5mm thick). The important part is the weight of the sugar syrup as you will use this syrup for an italian meringue.
Part 4: 250 grams icing sugar, 1 tbsp vanila extract, 300 grams unsalted butter cut into small pieces about 1-2 cm cube.
Make the berry butercream:
Part 1: heat the frozen berries and the rubhard 60 grams of sugar in a small pot, until everything is softened. Then purrée and and push through a fine seive. Get as much of the fruit through the seive as you can. If the end result is too runny, return to the put and reduce a bit longer, but not too long! You want the purée to have a bit of texture, and not be completely runny.
Part 2: in another small heavy pot, add the golden syrup, 120 grams of the sugar, and the vanilla, mix and heat, gently swirling the pot occasionally (you should be able to do this visually, when the mixture is bubling with big bubbles it is ready, but you are aiming for 115/116 degrees C.
While the sugar is cooking place teh 4 egg yolks into a stand mixer with the whisk attachment and beat on medium/high speed until thick and its colour has gone light yellow. When the syrup is at temperature, slow down the speed of the mixer and gently pour slowly the sugar over the egg yolks. Then slowly raise the speed of the mixer up to high speex and keep beating until the bowl is cool to the touch. You may speed this up by holding bags of frozzen peas or fruit against the bowl.
When the bowl is cool and the eggs are now at room temperature slowly add, a piece at a time the butter (300 grams). Make sure each piece is fully incorporated before adding the next one. Don't worry if it looks kind of funny at first. It will mix. After the butter has been added, add 80 grams of the fruit purée. Make sure everything is mixed, and put aside.
Icing number 2 (we need to do this in stages), and will start with part 3:
Heat up the sugar, water and rhubard slices. When the sugar has started to simmer, and is all disolved cook for a minute, then turn off the temperature and let the rhubard sit in the sugar syrup. You want the rhubhard to keep its shape, so don't cook for too long. The point is the sweeten the rhubard, and infuse the sugar with the rhubard.
Using a stand mixer with the paddle attachment, mix the butter and icing sugar. We are making an American Buttercream. If you are very good a making icing, and you want less sugar, you can skip this step and reduce the amount of butter to 250 grams. Start pulsing on very slow speed until the sugar is mixed, then turn up the speed and beat for a few seconds. Add the tbsp of vanilla extract and keep beating so that the mixture takes on a smooth texture. Save this for later.
Now, remove the rhubard with a slotted spoon and place each piece individually on a paper towel. We need to get rid of some of the moisture. Save the sugar syrup.
Turn on your oven to 180 degrees C (for a convection oven, or 200 if it isn't convection)
Line your baking tray with parchment paper (I use a bit of butter to help the paper stick. You need to make sure the paper goes over the edges of the tray.
In a stand mixer with a whisk attachment add the 8 egg yolks with the sugar and lemon juice from the lemon and beat on med/high until thick and pale in colour. Trasnfer this mixture to a large mixing bowl, and sift the flour and salt over the eggs. Then gently fold the flour into the egg mixture. When it is almost completely mixed, sprinkle the lemon zest on top. Now completely clean the stand mixing bowl as we need to beat the 8 egg whites. Add egg whites, and a few drops of lemon juice from the second lemon, and start beating using the whisk attachment (it too, must be completely clean). When the the egg whites start to form soft peaks, gently pour in the 20 grams of sugar and keep beating until stiff peaks are formed. The mixture should be firm and glossy. Do not overmix. Once the egg whites are ready, gently fold a third of the egg whites into the yolk/flour mixture, and then gently fold in the remaining egg whites.
Pour the mixture into the lined baking tray, and use a palette knife to even out the mixture. Bake for 12-15 minutes until the cake is lightly golden brown. You can use a toothpick to test for doneness -- it should come out clean.
When the cake is done, remove from the oven and place on a wire rack to cool for about 5 minutes. After the 5 minutes has passed, sprinkle the top of the cake with a good dusting of icing sugar (this is to prevent the cake from sticking to the kitchen towel). Place a kitchen towel over the cake, and if you've got a large tink cutting board or cookie sheet to use as a support, flip the cake over. If you've used the cutting board of baking sheet, gently slide it away. Gently peel off the parchment paper. Using the kitchen towel, roll the cake, but do not roll the towel inside. You are using the towel as a guide to help. Place the rolled cake back on the wire rack to cool.
Back to the icing!
Okay, while the cake is cooling, remove the rhubard from the paper towel (we don't them to stick), and place on a plate, ready to use. We are now going to get our other icing ingredients ready. We are going to make an italian meringue, and and then divide the meringue in half. Half of which we will use for the exterior icing, the other half for the top decoration.
We need to reheat the rhubard syrup, up to 115/116 degrees C. In a very clean stand mixer with the whisk attachment, add the 4 egg whites, and a few drops of lemon juice from our extra lemon. We need to beat these to stiff peaks and then gently pour in the heated syrup. The process here is the same that we did with the egg yolks. Slowly pour into the sugar syrup over the egg whites, and then raise the speed and beat until cool.
Once the egg whites are cool, remove half of the Italian meringue. And you want to gently and slowly, mix the American Buttercream or the plain butter into the remaining meringue, just as we did with the other icing, a piece or spoonfull at a time. The technique is the same.
Now we assemble:
Unroll the cake. Cut it in half, lengthwise (with the long ends facing you, cut parallel to the long side). Be precise, I measure, as we need each half to be exactly the same width. Line up with the small ends touching, so we now have one long piece of sponge that we can roll. Using a palette knife, spread the fruit buttercream along the cake. I use all of it. And then sprinkle with the candied rhubard. Roll the cake.
Take your serving dish, and dot with a bit of leftover icing in the centre of the plate. This to help the cake stick to the place. You don't need much. I spread a teeny amount using a rubber spatula. Place the cake on the the plate. You want to see the rolled layers from the top. Use some strips of parchment paper to create a border on the base on the plate around the base of the cake. This is to keep the plate clean as we ice the cake. Using icing number two, cover the cake, also using a palette knife.
At this point, you've got a nice completely iced sponge! Using the leftover berry purée, pout a bit along the top of the cake. What you ultimately want is a very thin layer along the top of the cake, with some icing dripping down the edges (see the photo -- you can also tell that my purée wasn't thick enough). You don't want much of this berry on top, as otherwise the meringues will slide.
Using a pastry bag, pipe little meringue dollops along the top of the cake, and then use the blow torch to lightly torch each meringue.
If you've got any leftover fruit purée and some of the cadied rhubard, use them as garnish when serving.