Trying to find icing that is close to what you can buy in the US is pretty hard when you live in Europe. So my wonderful wife actually looked up a recipe but wasn't happy with it so she modified it a bit. So here it is and it's pretty good.
4 Tablespoons butter
1/2 cup packed brown sugar
3 Tablespoons milk
1/2 cup powdered sugar
1/4 cup unsweetened cocoa powder
1 teaspoon vanilla extract
In a saucepan melt butter, stir in brown sugar. Continue stirring until sugar is dissolved, then add milk. Bring to a boil and remove from heat. Mix in powdered sugar and cocoa powder and vanilla extract. If the mixture seems too thick add a little more milk.
Spread on cooled cake rather quickly as the icing will set quickly.