Cupcakes with Chocolate Buttercream Swirls

For Chocolate Cupcakes:

Position oven rack in the middle. Preheat oven to 350º F / 180º C. Line standard 12 hole muffin tin with paper or foil liners.

Using an electric mixer set on high speed, cream together butter and sugars until light and fluffy, about 5 minutes. Lower the speed to medium, add the vanilla and the eggs one at a time and mix well.

In a separate bowl, whisk together the buttermilk, sour cream, and coffee. In a separate bowl, sift together the flour, cocoa powder, salt and baking soda. With the mixer on low speed, starting with the buttermilk mixture and ending with the flour mixture, add alternately in thirds to the butter and sugar mixture. Mix until just blended, scraping the sides of the bowl with a rubber spatula to completely incorporate.

Divide batter evenly between the lined muffin cups, filling each cup three-quarters full. Bake for 18 to 22 minutes or until a cake tester inserted in the center comes out clean. Rotate tin halfway through the baking process to promote even baking. Cool for 10 minutes in the pan. Remove cupcakes from pan and transfer onto wire racks to cool completely before frosting.

For Chocolate Buttercream Frosting:

Place the chopped chocolate in the top pan of a double boiler set over but not touching simmering water. Stir constantly until the chocolate is melted and smooth. Let cool to room temperature.

In the bowl of an electric mixer fitted with the flat beater, combine the confectioners' sugar, butter, milk, vanilla and salt; beat on low speed until combined, about 1 minute. Stop the mixer and scrape down the sides of the bowl. Increase the speed to medium and beat for 2 minutes, then reduce the speed to low. Add the melted chocolate and beat until combined, increase the speed to medium and beat for 1 additional minute.

If the frosting appears dry, add more milk, 1 teaspoon at a time, until desired consistency is reached.