Preheat the 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 beer mixture, then flour mixture; beat until 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, about 2-3 minutes.

Set mixer to low speed and slowly beat in confectioners' sugar, one cup at a time, until frosting is smooth and spreadable. Beat in Irish cream liqueur; adjust the thickness of frosting with more confectioners' sugar if needed.