1 cup Irish stout beer (such as Guinness®)
1 cup butter
3/4 cup unsweetened cocoa powder
2 cups all-purpose flour
2 cups white sugar
1 1/2 teaspoons baking soda
3/4 teaspoon salt
2 large eggs
2/3 cup sour cream
2/3 cup heavy whipping cream
8 ounces bittersweet chocolate, chopped
2 tablespoons butter
1 teaspoon Irish whiskey, or more to taste
1/2 cup butter, softened
3 cups confectioners' sugar, or more as needed
3 tablespoons Irish cream liqueur (such as Baileys®), or more to taste
Preheat oven to 350 degrees F (175 degrees C).
Line 24 muffin cups with paper liners.
Bring Irish stout beer and 1 cup butter to a boil in a saucepan and set aside until butter has melted, stirring occasionally. Mix in cocoa powder until smooth.
Whisk together flour, sugar, baking soda, and salt in a bowl until thoroughly combined.
Beat eggs with sour cream in a large bowl with an electric mixer on low until well combined. Slowly beat in the beer mixture, then the flour mixture; beat until the batter is smooth.
Divide batter between the prepared cupcake cups, filling each cup about 2/3 full.
Bake in the preheated oven until a toothpick inserted into the center of a cupcake comes out clean, about 17 minutes.
Cool the cupcakes completely. Cut cores out of the center of each cupcake with a sharp paring knife. Discard cores.
Bring cream to a simmer in a saucepan over low heat; stir in bittersweet chocolate until melted.
Mix in 2 tablespoons butter and Irish whiskey until butter is melted; let mixture cool to room temperature. Filling will thicken as it cools.
Spoon the filling into the cored cupcakes.
For frosting, whip 1/2 cup butter in a bowl with an electric mixer until fluffy, 2 to 3 minutes.
Set mixer to low speed and slowly beat in confectioners' sugar, 1 cup at a time, until frosting is smooth and spreadable. Beat in the Irish cream liqueur; adjust thickness of frosting with more confectioners' sugar if needed.
Spread frosting on filled cupcakes.