lightly grease the tins and line the bases with non-stick baking parchment. Pre-heat the oven to 180C/350F/Gas 4.
Measure the butter or margarine, sugar, eggs, flour, baking powder and cocoa into a large bowl and beat for about two minutes, until just blended; an electric mixer is best for this, but of course you can also beat by hand with a wooden spoon.
Divide the mixture evenly between the prepared tins and level the surface with the back of a spoon or a plastic spatula.
Bake for about 25 minutes, until well risen and golden. The tops of the cakes should spring back when pressed lightly with a finger. leave the cakes to cool in the tins for a few minutes, then run a small palette knife or blunt knife around the edge of the tins to fee the sides of the cakes. Turn the cakes out on to a wire rack, peel off the paper and leave to cool completely.
Choose the cake with the best top, then put the other cake top downwards on to a serving plate. Spread with the jam and whipped cream, put the other cake on top (top upwards) and sprinkle with caster sugar to serve.