One of our favorites is right on the back of the Hershey's Cocoa can:
"Perfectly Chocolate" Chocolate Frosting
1 stick of butter softened (recipe calls for melting, but I prefer this way)
3 cups powdered sugar
2/3 cup cocoa
1/3 cup milk
1 tsp vanilla
Beat butter, powdered sugar, and cocoa together. It is best to add a cup of powdered sugar at a time and beat (add some of the milk if it gets too stiff). Add milk and vanilla. Beat until smooth. You can add more milk if needed. Makes about 2 cups of frosting.
My MIL thinks it compares to canned frosting, but I think it's wayyy better!
Here's another:
Chocolate Butter Cream
1/2 cup softened butter
1/2 cup shortening
4 cups powdered sugar
3/4 cup cocoa
1/4 cup milk -- may need more
1tsp vanilla
Beat butter and shortening together until creamed together. Add powdered sugar and cocoa one cup at a time. You can sift your powdered sugar and cocoa before adding to the mixure, but honestly I don't. Beat well after each addition (can add some of the milk if it gets too stiff). Add milk and vanilla beat well. You can also add more milk for desired consistency. Remember to add a little at a time -- you can always add more, but can't take excess away.