This classic dessert may be prepared ahead of time. Fill with vanilla cream, sprinkle with icing sugar, then cut into slices to serve.
Preheat the oven to 220 degrees Celsius/200 degrees Celsius fan/gas 7. Line a 22 x 32cm swiss roll tray with baking paper (a 20 x 30cm tin would suffice). Sift together the flour and cocoa powder in a mixing dish and put aside.
Add the sugar to the eggs in a separate dish or the bowl of a stand mixer. For 5 minutes, beat the eggs and sugar together with an electric whisk or the whisk attachment until pale and extremely thick. Gently fold in the flour mixture with a spoon or spatula until it is completely mixed and there are no streaks.
Scrape the mixture into the tin gently and spread it out to the corners. Preheat oven to 350°F and bake for 9-10 minutes, or until puffy and springy to the touch. While the cake is baking, prepare a second piece of parchment that is slightly larger than the tin and sprinkle it with sugar. When the sponge is done, carefully flip it onto the sugar-coated paper using oven gloves. Remove the pan from the oven and peel away the top layer of parchment paper. Score a line 2cm from one of the shorter sides, then coil the sponge into a spiral with the parchment running through it, using the paper as a guide. Allow the rolled sponge to cool fully before using.
Make the filling while the cake is cooling by whisking together all of the ingredients until the cream stiffens. Unroll the sponge and distribute it over the filling, being sure to leave the scored edge clean. Using the parchment, roll the sponge up around the filling, starting at the scored edge. Dust with icing sugar and place on a board or serving plate. Serve immediately or keep refrigerated for up to 24 hours. Cut into pieces and serve.