This frosting (which I got from Bloghungry) is easy to make and amazingly rich and chocolatey. Oh man. So good. Note: this frosting needs time to set up before it is ready to eat, so it's best to frost your cake the day before you want to serve it.
4 C powdered sugar
1 C cocoa powder
1 stick (½ C) butter
12 large marshmallows
2/3 C milk
2 tsp vanilla extract
Sift sugar and cocoa together into a large bowl and set aside. Place marshmallows, butter, and milk in a medium sized sauce pan over low heat. Stir marshmallows until melted, which takes a long time. Remove from heat and combine with sugar mixture. Add vanilla and stir until smooth and satiny. Use at once to spoon over cake or cup cakes. Frosting will be extremely thin -- about the consistency of cake batter -- but it will firm up as it cools.