Here is a chocolate cake recipe that I adapted.  My mum uses cocoa but it's quite expensive so I used drinking chocolate and changed the recipe.
It's nothing special but it tastes nice and is useful for making flatmates like you.

Ingredients
6 oz drinking chocolate (the stuff you add to hot milk)
3 oz sugar
5 oz self-raising flour
6 oz margarine
3 eggs

Method
Preheat oven to approx 180 C/Gas mark 6.
Grease and line cake tin.
Sieve flour and drinking chocolate into a bowl.
Add sugar, eggs and margarine.
Stir until very smooth and nice.
Pour into the tin and bake for about 30 minutes (check after 20, then after 5, then after 5, then as it gets more ready check more often.)
To test if it's baked, stick a fork in.  It should come out clean from most of the cake but right in the middle it should be a bit sticky.
Leave cake (still in tin!) on a wire rack until cool.
Remove cake from tin.
Top with melted plain chocolate cake-covering.