Preheat oven to 180 degrees C and line a baking tray with baking paper.
Sift together the flour, cocoa and cornflour.
Set a saucepan of water at a simmer and place the mixing bowl over the water, making sure water does not touch bowl. In the bowl, beat the eggs and sugar with an electric beater until doubled in volume. Remove from the water bath and keep beating until cool. Stir in the flour mixture.
Pour mixture over the baking tray and smooth with a spatula. Bake 8 minutes, checking the batter does not harden.
Remove from the oven, fill as desired and roll up.