For the Christmas Sponge Roll, crack the eggs into a mixing bowl. Add sugar and vanilla sugar and mix until fluffy.
Preheat the oven to 210°C (fan oven 180°C). Sift the baking powder with the flour and fold into the sugar mixture.
Save a small amount of the dough and stir the cocoa into it.
Line a baking tray with baking paper and spread the light dough mixture evenly and smooth it out. Pour the cocoa mixture into a piping bag and use it to pipe individual large dots onto the sponge mixture. Use a toothpick to pull these dots apart in a star shape. Bake for approx. 15 minutes.
In the meantime, place a clean tea towel on top and lightly sprinkle with sugar. Turn the finished sponge cake upside down on it. Spread with jam and carefully roll up into a sponge roll with the help of the tea towel.