Source: Cooking Books
Chocolate Cake with Chocolate Buttercream Frosting
Adopted from Buttercup Bakes At Home
For the Cake
2 cups all-purpose flour
1 teaspoon baking soda
1/2 teaspoon salt
1 cup unsalted butter, softened
1 cup sugar
1 cup firmly packed light brown sugar
4 large eggs, room temperature
6 ounces unsweetened chocolate, melted (easy to do in the microwave)
3/4 cup buttermilk
1 teaspoon vanilla extract
1/4 cup sour cream
Oven preheated to 350. Grease two 12-cup muffin tins for cupcakes, or line with cupcake papers if you're using them. In a medium bowl, whisk together the flour, baking soda and salt, then set aside.
In the bowl of an electric mixer, or with a hand-held mixer, cream the butter and sugars together until light and fluffy. This should take about 2-3 minutes. Add the eggs one at a time and beat after each egg until they're incorporated. Add the flour mixture in thirds and alternate with the buttermilk and vanilla. Beat between each addition. The mixture should be smooth once everything is added. Stir in the sour cream, and make sure any ingredients on the bottom of the bowl have been incorporated.
Fill the muffin cups to about 2/3rds full. Bake for 20-22 minutes, until the tops spring back when poked with your finger. Allow to cool for 10 minutes before removing the cupcakes to a cooling wrack. Cool completely before frosting.
For the buttercream
1 1/2 cups unsalted butter, softened
2 tablespoons milk
9 ounces semisweet chocolate, melted and allowed to sit until lukewarm
1 1/2 teaspoons vanilla extract
3 cups powdered sugar
In the bowl of an electric mixer, or with a hand-held mixer, beat the butter until light and creamy. This should take about 2 to 3 minutes. Add the milk in a thin stream and continue beating until smooth. Add the melted chocolate, then add the vanilla and beat for another minute or so. Gradually add the sugar and beat until it reaches a creamy, wonderful, shove-your-face-into-it consistency. Taste as you go so it's exactly as sweet as you want it to be.
You should store the buttercream at room temp, otherwise it will get hard in the fridge. Use it to frost your cupcakes!