Cream the sugar and butter into a large bowl, beat until light and pale. Add the eggs one at a time and continue to beat until you leave a trail on the surface when the whisk is lifted (with an electric mixer, about 5 mins).
Sift the flour. Gently fold half the flour into the mixture. When it is well combined, fold in the other half of the flour along with the cocoa powder and baking powder. Stir in the vanilla extract.
Turn the mixture into the prepared tin.
Bake the chocolate sponge cake for about 30-35 mins or until a cake skewer inserted in the middle comes out clean.
Remove from the oven and leave to cool. Turn out onto a wire rack and remove the greaseproof paper.
Turn the chocolate sponge cake upside down so the top becomes the bottom. Slice it horizontally and fill with whipped cream and fruit - raspberries or strawberries work well. You can decorate the top with more fruit.
Or, for die hard chocoholics, make our quick, chocolate icing. Use an electric whisk to beat together all the ingredients until you have a smooth, light icing. Use half of it in the middle of the chocolate sponge cake and the other half on top. Decorate with raspberries if you fancy.