I like the combination of raspberry and chocolate as it prevents the cake being too sweet. Originally I saw the recipe in delicious! magazine: chocolate and berry tray bake
I modified it a bit to make it more chocolatey (and easier to remember) and added extra baking powder to lighten it.
250g caster sugar
4 eggs (I used 5 coz they were small)
2 tsp vanilla essence
250g self raising flour
2 tsp baking powder
250g drinking chocolate
250g frozen raspberries
Cream the butter and sugar together until light and fluffy then beat in the eggs. Mix in the dry ingredient, then the milk (to make a nice smooth batter) and the raspberries. Pour into a lined and greased baking tray (I used a roasting dish) and bake at 160 degrees Celsius for 50 mins until springy. Cool in the tin a bit then on a wire rack.
I decorated it with some melted chocolate and mushed up raspberries (and birthday candles).