For the cake
For the filling:
Pre-heat oven to 220C/Fan 200C/Gas 7. Grease a 33 x 23cm (13 x 9 in) Swiss roll tin and line with baking parchment.
Whisk the eggs and sugar in a large bowl until the mixture is light and frothy and the whisk leaves a trail when lifted out. Sift the flour and cocoa into the mixture, carefully folding them in at the same time. Turn the mixture into the prepared tin and give it a gentle shake so that the mixture finds its own level, making sure that it spreads evenly into the corners.
Bake for about 10 minutes or until the sponge begins to shrink from the edges of the tin. While the cake is cooking, place a piece of baking parchment a little bigger than the size of the tin onto a work surface and sprinkle it with caster sugar.
Invert the cake onto the sugared parchment. Quickly loosen the parchment on the bottom of the cake and peel it off. Trim the edges of the sponge with a sharp knife and make a score mark 2.5cm (1 in) in from one shorter edge, being careful not to cut right through. Roll up the cake firmly from the cut end, with the parchment inside, and leave to cool.
Warm the jam gently in a small pan only until it is of a consistency that is easy to spread. Carefully unroll the cooled cake. Remove the parchment, spread with jam and whipped cream and re-roll.