Step 1: Make the cupcakes
1 cup (2 sticks) unsalted butter
3/4 cup unsweetened cocoa powder
2 cups all purpose flour
2 cups sugar
1 1/2 teaspoons baking soda
3/4 teaspoon salt
2 large eggs
2/3 cup sour cream
Preheat oven to 350°F. Line 24 cupcake cups with liners. Bring 1 cup stout and 1 cup butter to simmer in heavy large saucepan over medium heat. Add cocoa powder and whisk until mixture is smooth. Cool slightly.
Mx flour, sugar, baking soda, and salt in large bowl to blend. Using a mixer, beat eggs and sour cream in another large bowl to blend. Add stout-chocolate mixture to egg mixture and beat just to combine. Add flour mixture and beat briefly on slow speed. Using rubber spatula, fold batter until completely combined. Divide batter among cupcake liners, filling them 2/3 of the way. Bake cake until tooth pick inserted into center comes out clean, bake about 17 minutes. Cool cupcakes on a rack completely.
Step 2: Make the filling
2/3 cup heavy cream
2 tablespoons butter, room temperature
1 to 2 teaspoons whiskey (optional)
Chop the chocolate and transfer it to a heatproof bowl. Heat the cream until simmering and pour it over the chocolate. Let it sit for one minute and then stir until smooth. Add the butter and whiskey (if you’re using it) and stir until combined.