Really, what goes better with yellow cake than chocolate frosting? It's a classic! This combo is The Physicist's favorite. Once again, this is adapted from Martha.
1/2 pound Really Good Chocolate (Dark or Semi-Sweet)
3 Tbsp. Dutch-Process Cocoa Powder
3 Tbsp. Boiling Water
1 1/2 sticks Unsalted Butter, Room Temp
1/4 cup Powdered Sugar
Pinch of Salt
Okay, first we melt the chocolate by placing it in a heat proof bowl (like Pyrex), over, but not touching, some simmering water in saucepan. This is pretty much making your own double boiler. Turn off the heat, and stir until the chocolate melts completely. Set this aside and let cool to room temp. While your waiting, combine the cocoa and the boiling water until the cocoa is dissolved. (It is important that both the chocolate and the cocoa mixture cool to room temp, or you'll melt your butter!)
Once everything is room temp, place your butter, powdered sugar, and salt in the mixing bowl of your electric mixer and beat until light and fluffy with the paddle attachment for 3-4 minutes. (No mixer? Get your wooden spoon and work those arms!) Add the melted chocolate, beat until combined. Then add the cocoa mixture. Scrape down the sides, and beat until smooth.