A chocolate Ganache would be yummy. Equal parts cream and chocolate.

Or here is a choc icing i lifted from another forum J.FOR THE TOPPING

50gr plain chocolate
2 tbsp water
25gr butter
175gr icing sugar

melt the chocolate and butter together with the water in a heatproof bowl over a saucepan of gently simmering water. Sift in the icing sugar and beat until smooth and thick. spread the warm icing on top of each cupcake and leave to set before serving

I believe Niliac has an amazing butter icing recipe..... i'm sure it's in this thread somewhere... hmmmm