Preheat the oven to 220℃/200℃fan/425℉/Gas 7. Grease and line a 33 x 23 cm Swiss roll tin with baking paper.
Whisk the eggs and sugar in a large bowl until light and frothy (2-3 min). Sift the flour and cocoa into the mixture and fold in gently. Turn into the tin and level out. Bake in the oven for 10 minutes.
Meanwhile, place some baking paper (bigger than the tin) onto a work surface and sprinkle with caster sugar.
Invert the cake onto the sugared paper. Gently peel off the baking paper that's aready on. Trim the edges of the sponge with a sharp knife and score mark 2.5cm from one shorter edge but don't cut right through. Roll up the cake firmly from the cut end, with the paper inside and leave to cool.
If you are using jam, warm it up in the microwave so it spreads easily. Make sure it's not too hot, or it will soak the sponge.
Unroll the cake, remove the paper, spread with jam and whipped cream and re-roll.
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