Make a 'normal' cake with plain flour and 1 tsp baking powder for each egg using say 3 eggs, 180g plain flour, 3tsp baking powder, 180g castor sugar and 180g of melted plain chocolate.
First you separate the eggs and whisk the whites until stiff and reserve.
You then mix the butter and sugar and add the melted chocolate.
Add the eggs yolks and then fold in the flour/baking poweder sifted together and the whisked egg whites.
Cooks at whatever temperature you use for standard Victoria sponge.
Normally you add 3/4 tsp baking powder for each egg, but because chocolate makes the mixture heavier you need 1tsp - that's why you can't really use self-raising flour.
You can fill it with home-made lemon curd, lemon curd or raspberry jam and whipped cream, choc truffle layer with Cointreau soaked raspberries and a whipped cream layer. Just depends on what you fancy.
Choc truffle - melt 200g ie large bar of good plain choc, add a slosh of double cream and whip with a balloon whisk until it's the colour of dark milk choc. Slather over cake with or without alcoholic raspberries.
Optional icing - proportions given, but may need double quantity for a larger cake
Melt 150g plain chocolate with 65g butter. Mix well and allow to cool a little. Then either pour over cake to form a shiny coating, or whisk with a balloon whisk until paler in colour, and trowel on with a palette knife - forms a sort of frosting.
Dd picked at this on a cake when she was about 7 - it's known as picky-cake icing for obvious reasons.