I found this recipe a couple of years ago online and have been using it ever since, any time I need chocolate frosting. It is the best chocolate frosting I've ever had...it tastes like homemade chocolate ice cream. I usually double the recipe...
*Tip* If you want a deep black color like I have on the secret agent cake or Star Wars cheesecake shown in this blog, it is better to start with a chocolate frosting like this rather than going from a white frosting (will take much less color). My favorite cake colorings are the gel paste colors sold by Americolor. I particularly love their "super black" and "super red" because I can't detect a taste, they come in a plastic bottle with a flip top lid, and are far less messy and troublesome than the Wilton cake colors. You can order Americolors online at the pastry shop websites...they make really awesome colors to pick from.
1 stick butter
1/2 cup cocoa
4 cups confectioner's sugar
~1/4 cup milk
1. Beat butter on high for about 30 seconds, until soft.
2. Add cocoa and 1 cup of sugar and beat until incorporated.
3. Add half of the milk and the remainder of the sugar and beat until well incorporated.
4. Continue to add milk until you get to the consistency you want.