Ingredients

Step by step

1. Preheat the oven to 180°C, fan 160°C, gas 4 and line a large Swiss roll tin, about 28cm x 39cm, with baking paper.
2. You’ll need to make and bake the sponge in two batches: put 5 eggs in a free-standing mixer with 150g sugar and whisk until doubled in size; it will take around 5 minutes. Carefully fold in half each of the flour and ground pistachios, and then half the melted butter. Pour into the prepared tray and bake for 12-14 minutes, or until lightly golden and springy to the touch. Remove from the oven and leave to cool in the tray for 2 minutes.
3. Lay a sheet of baking paper on your worktop, the same size as the tin. Sprinkle with 1 tablespoon of the extra caster sugar and then invert the tray onto the paper. Carefully remove the lining paper, then lay a clean sheet of baking paper on top. Carefully roll up the sponge like a Swiss roll from one of the short sides (with the paper inside) and leave to cool completely. Repeat the steps with the remaining ingredients to create another cake.
4. To make the buttercream, melt the white chocolate in 30-second bursts in the microwave; set aside to cool. Put the egg whites in the bowl of a free- standing mixer with the whisk attached. Put the sugar in a pan with 50ml of water and heat gently until the sugar has dissolved. Bring to a simmer and cook until it reaches 116°C on a cook’s thermometer, then start to whisk the egg whites.
5. When the sugar syrup reaches 120°C and the egg whites have soft peaks, slowly drizzle the hot syrup into the egg whites while whisking. Continue to whisk until the meringue is completely cold; this usually takes around 10 minutes.
6. Slowly add the butter to the meringue, one chunk at a time, until it has all been incorporated. The mixture will split during this process, but it will all come together after about 5 minutes. Mix in the cooled chocolate, then add the rose water until you get the strength of flavour that you want.
7. Unroll one sheet of sponge and cut it in half lengthways. Spread both pieces of sponge with buttercream, then roll up one half tightly. Abut the end of the unrolled sponge with the end of the rolled-up sponge and then continue to roll so that you’re creating a fat roll that will eventually stand on its end. Repeat with the second sponge, abutting the ends to give one large roll, and then place this with the swirl side down on a cake board. Cover the top and sides with the remaining buttercream and decorate with chopped pistachios and flowers.