Line muffin tin with paper liners. (Or you can use pan spray if you prefer, but I find the liners work better). I used a 12 muffin pan.
Preheat oven to 350F
Melt: 2 oz chocolate or 1/4 C dark or bittersweet chocolate chips. Set aside. (I use the microwave to do this; it takes about a minute on high).
Mix in one bowl:
1/4 C cocoa
1/4 tsp baking soda
1/4 tsp baking powder
1/4 tsp salt
Cream in separate bowl:
1/2 C (one stick) butter
3/4 C sugar
1 egg yolk
Mix thoroughly. Add 1/2 of the cocoa flour mixture, then add:
1/2 C buttermilk (plain yogurt also works)
Mix, then add the remaining flour mixture. Once this is blended, add in melted chocolate and mix until blended. If desired add:
1 C chocolate chips (not melted)
Divide the batter evenly in muffin tin, and bake for 20-22 minutes, or until a knife inserted into the center of a muffin comes out clean. Remove muffins from tin and set to cool on a cooling rack. Cool muffins. If you're in a rush, stick them in the fridge for ten minutes or so.
Irish Cream Buttercream frosting:
It seems silly to even write out this recipe since it's so basic, but here's what I did:
1/4 C butter
1 1/3 C powdered sugar.
Roughly 2 tbsp Bailey's Irish cream. Basically you add Irish cream until you have the consistency you want. If you overshoot add a little more powdered sugar. Whip the heck out of the frosting until it looks kind of fluffy, then spread over the top of cupcakes.
(Next step: learn how to pipe frosting in a properly Martha Stewartesque manner).