Lemon Berry Stripe Cake

PURÉE:

1. Place the mixed berries and granulated sugar in a medium saucepan and place over medium-low heat. Cook for for 5-7 minutes, until the berries have softened, the sugar has dissolved and everything has cooked a little.
2. Transfer to a blender and pulse to a purée. Strain through a fine-mesh sieve set over a bowl to catch the purée; you only need 2/3 cup, so save any extra in the fridge for another use.

CAKE:

1. Preheat the oven to 400°F. Line a shallow 12- by 17-inch rimmed baking sheet with parchment paper and set aside.
2. Place the egg yolks in the bowl of an electric mixer with the whisk attachment in place. Add the 2/3 cup plus 2 tablespoons granulated sugar and the lemon juice and beat on medium-high speed for about 3 minutes, until pale and thick. Transfer the mixture to a large mixing bowl and sift the flour and salt directly over the egg mixture in two batches, folding through the mixture with a rubber spatula after each addition. Sprinkle the lemon zest on top and set aside.
3. Place the egg whites in a clean bowl of an electric mixer with the whisk attachment in place. Whisk on medium-high speed until soft peaks form, then slowly pour in the 1½ tablespoons granulated sugar. Continue to whisk until firm peaks form, then gently fold a third of the egg whites into the egg yolk mixture until incorporated. Finally, fold in the remaining egg whites until combined, and then scrape the mixture into the lined baking sheet. Even the surface out with a small spatula and bake for 12-14 minutes, or until light golden brown and a toothpick inserted into the centre comes out clean.
4. Remove from the oven and place on a wire rack to cool in the pan for 5 minutes before dusting the top lightly with powdered sugar. Place a clean kitchen towel on top of the sponge cake and then flip it over so that it is now lying on top of the kitchen towel. Carefully peel away the paper and trim the very edges of the sponge. Starting at the shorter edge of the cake, carefully roll it up (along with the kitchen towel). This is to "train" the cake, ready for rolling up again later. After about 20 minutes, or when no longer warm, unroll the cake. With the short end facing you, measure and cut down the centre of the cake, parallel to the long edge, so that you have two strips, each about 5.5 inches wide. Cover with a clean kitchen towel and set aside.

BUTTERCREAM:

1. Place the corn syrup, granulated sugar and vanilla seeds in a medium saucepan. Place over low heat and stir until all the sugar dissolves; this is your sugar syrup.
2. While the syrup is cooking, place the egg yolks in the bowl of an electric mixer with the whisk attachment in place. Beat on medium-high speed until thick and pale yellow in colour. Leave the machine on while you check the sugar syrup; when all the sugar has melted, stir again, increase the heat to medium and simmer until bubbles begin to appear. Swirl the pan gently and continue to simmer until there are large bubbles all over the surface of the syrup.
3. Remove the pan from the heat and carefully pour the hot syrup in a slow, steady stream down the edge of the mixing bowl into the beating yolks. When all the syrup has been added, increase the mixer speed to high and continue to beat the mixture for about 10 minutes, until the outside of the bowl is no longer warm. Gradually add the butter, one cube at a time, allowing it to be incorporated into the mixture before adding the next. When all the butter has been added, scrape down the bowl and continue to beat for another minute, until the buttercream is very smooth and light. Add a scant 1/2 cup of the berry purée and beat on medium speed until fully incorporated.

ASSEMBLY:

1. Spread each of the strips of sponge with about a quarter (5 ounces) of the buttercream; this should leave half (about 10 ounces) to frost the top and sides of the cake. Take one strip of sponge and, starting with the short end, roll it up. Once this strip is rolled, position the exposed end at the beginning of the second strip and continue to roll. You now have a rolled cylindrical cake! You should end up with a coiled barrel shape. Turn the cylinder onto the cake stand so that it is standing on one of its flat ends.
2. Spread the remaining buttercream all over the top and sides of the cake, smoothing with a spatula to create an even surface. Chill for 10 minutes to set the frosting, then decorate with fresh flowers or berries. Set aside for 1 hour at room temperature before serving.