1. Preheat oven to 350 degrees F. Line 12 muffin cups with paper liners. Spray liners and flat top around each cup with nonstick vegetable spray; these are oversized cupcakes that will rise above surface of cups.
2. Make cupcakes. Sift flour, cocoa powder, baking soda, and salt into a medium bowl. Set aside.
3. In a large bowl, use an electric mixer on medium speed to beat 6 tablespoons of butter and sugar until fluffy and lightened in color, 2 minutes. Using a rubber spatula, scrape down sides and bottom of bowl and beaters as needed. Add egg, egg yolk, and 1 teaspoon vanilla extract and beat until smoothly blended, 2 minutes.
4. Add 1/3 of flour mixture, mixing just until flour is incorporated and batter looks smooth. Add half the buttermilk, and then repeat with remaining flour and buttermilk, ending with flour. Stir just until batter looks smooth.
5. Divide batter evenly among 12 lined muffin cups. Bake just until a toothpick inserted in center of cupcake comes out clean and top feels firm, about 21 minutes. The tops may have a few small cracks. Cool cupcakes for 10 minutes in pan on a wire rack. Transfer cupcakes in their paper liners to wire rack to cool completely, about 30 minutes.
6. Make filling. In a large bowl, use an electric mixer on low speed to beat remaining 6 tablespoons of butter, confectioners' sugar, and 1 teaspoon of vanilla extract together with enough milk to form a creamy, smooth, spreadable filling.
7. When cupcakes are completely cooled, use a small knife to cut out a cone-shaped piece about 1 inch deep from top of each cupcake. Set aside. To prevent crumbling, let the cupcakes cool thoroughly before cutting out the center cone that will hold the filling. Fill opening in each cupcake with a heaping tablespoon of filling. Replace cupcake cone (it will sit above top of cupcake). Sprinkle lightly with confectioners' sugar. Cupcakes can be covered and refrigerated for up to two days.
Source: Hannaford Fresh Magazine, March - April 2007