Preheat oven to 350 degrees. Line 2 12-cup cupcake tins with cupcake liners.

Using a double boiler, add dark chocolate and butter over medium-high heat. Stir constantly until the chocolate is melted. Stir in the chocolate stout.

In the bowl of a stand mixer, beat sugar, eggs and yolks until light and fluffy, approximately 3 minutes. Add oil and sour cream, beating until combined. Pour in the chocolate mixture and beat until well combined.

In a separate medium bowl, whisk together flour, baking powder, cocoa powder, and salt. Add the dry ingredients into the wet ingredients, beating until just combined.

Pour batter evenly into the two cupcake tins. Bake for 18 to 20 minutes, or until a toothpick comes out clean when inserted in the center.

Cool completely before removing from pans.

While the cupcakes are cooling, make the frosting. Beat butter for the frosting on high until well combing, approximately 3 minutes. Add the remaining frosting ingredients, beating until well combined.

Frost cupcakes as desired.