Separate eggs. Beat the egg whites until stiff, then pour in 100 g of sugar. Stir in egg yolk. Mix flour, starch, 15 g cocoa and baking powder, sieve over the mixture and fold in carefully. Line a baking tray (32 x 39 cm) with baking paper. Put the mixture on top and smooth it down. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 8 minutes.
Sprinkle a clean tea towel with sugar. Turn the sponge plate over onto the cloth and remove the paper. Roll up the sponge from the long side with the tea towel. Let it cool down.
Whip the cream until stiff, adding cream firming agent and 1 tablespoon of sugar. Fold in the grated chocolate. Unroll the sponge cake. Spread the cream on top. Peel the banana and place it on the inner long side of the sponge cake roll and roll it up again. Chill for about 1 hour. Sprinkle the roll with 1 tbsp. cocoa and a little sugar before serving