From Smitten Kitchen who adapted it, barely, from a Sky High recipe
6 ounces unsweetened chocolate, melted and cooled
4½ cups confectioners’ sugar (no need to sift)
3 sticks (12 ounces) unsalted butter, at room temperature
6 tablespoons half-and-half or whole milk
1 tablespoon vanilla extract
Preheat oven to 350 degrees. Spray two 9-inch round cake pans with cooking spray, or butter them. Line bottom of pans with circles of parchment paper, then butter parchment.
Sift together flour, baking powder, baking soda, and salt in a medium bowl. In a stand mixer, beat butter and sugar with the paddle attachment at medium speed until pale and fluffy, then beat in vanilla. Add eggs 1 at a time, beating well and scraping down the bowl after each egg. At low speed, beat in the buttermilk until just combined (batter will look curdled). Add the flour mixture in three batches, mixing just until each addition is incorporated.
Divide batter evenly between the two cake pans, then drop pans on counter several times to get rid of any air bubbles. (This part makes a ton of noise and makes my girls scream and giggle!)
Bake the cakes until golden brown and a toothpick inserted into the center comes out clean, 35 to 40 minutes. Cool pans on a rack for 10 minutes, then run a knife around the edge of the pans. Invert onto rack and discard parchment, then cool completely, about 1 hour.
Place all of the ingredients in a food processor and pulse to incorporate, then process until the frosting is smooth.
Recipe by Kara In the Kitchen at http://karainthekitchen.com/homemade-yellow-cake-with-chocolate-fudge-frosting-how-to-frost-a-layer-cake/