Preheat the oven to 180 C / Gas 4. Grease a 33×23 cm (13 × 9 in) Swiss roll tin and line with baking parchment.
Use electric beaters to cream the eggs and sugar until the mixture is light and fluffy. Fold in flour and baking powder. Mix thoroughly. Pour cake mixture into the lined baking tin and tap gently to spread evenly.
Bake for 10–12 minutes or until the sponge is well risen and pale golden, and springs back when pressed gently with your finger.
Turn the sponge out onto a sheet of baking paper slightly larger than the sponge. Peel off the lining paper. Trim the crusty edges of the sponge with a sharp knife, if necessary. Roll up loosely from the short side, with the paper inside, and place on a wire rack to cool.
For the filing: Beat mascarpone cheese, caster sugar and coffee until well mixed.
When the sponge is cool, carefully unroll it and remove the paper. Spread the filling evenly over the cake and sprinkle with grated chocolate. Carefully roll up the sponge and place seam side down on a serving plate.
Refrigerate for 2 hours so the cake can absorb the flavours from the filling.