3 large eggs
1/4 cup unsweetened cocoa powder, plus extra for dusting
6 tbsp granulated sugar
5 tbsp butter, at room temperature
1/2 cup all-purpose flour
1 cup confectioners' (powdered) sugar, sifted, plus extra for dusting (optional)
Preheat the oven to 400 F (200 C). Place a large bowl over a pan of hot water, add the eggs and sugar, and beat with an electric mixer for 5-10 minutes, until the mixture is thick and creamy. Sift in the flour and cocoa powder and fold in with a spoon.
Line an 8 x 12in (20 x 30cm) jellyroll pan with parchment paper, then pour the mixture into the pan and level the top. Bake for 10 minutes, until the cake is springy to the touch. Remove from the oven, cover with a damp tea towel, and leave to cool.
Turn the cake out onto a sheet of parchment paper dusted with cocoa powder. Put the butter in a mixing bowl and beat with an electric mixer until creamy. Whisk in the confectioners' sugar a little at a time, then spread the mixture over the cake.
Using the parchment paper to help you, roll the cake into a long cylinder, starting from one of the short sides. Dust with more cocoa powder, if desired, and serve.