Here's a simple one from Julia Child. It's basically chocolate, butter and a little rum.
Soft Choclate Icing
(This is for a one-layer cake. You might want to double it for a larger cake.)
2 oz. sweet chocolate (ie. Baker's German's)
1 oz. unsweetened chocolate
(I've used 3 oz. bittersweet, total, but the combination is better)
1-1/2 Tbs. rum (or strong coffee)
A pinch of salt
3 oz. (3/4 stick) butter, room temperature, cut into pieces
Bring an inch of water to boil in a skillet and remove from heat. Combine the chocolates, rum and salt in a covered saucepan and set in the hot water for 5-10 minutes, until the chocolate is smoothly melted. Remove from the hot water and whisk in the butter one piece at a time until smooth.
Now put ice water in the skillet, return the frosting pan to it and whisk until it firms up to speading consistency.