Step 1
Preheat oven to 350 degrees F (175 degrees C).
Step 2
Line 24 muffin cups with paper liners.
Step 3
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.
Step 4
Whisk together flour sugar baking soda and salt in a bowl until thoroughly combined.
Step 5
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.
Step 6
Divide batter between the prepared cupcake cups filling each cup about 2/3 full.
Step 7
Bake in the preheated oven until a toothpick inserted into the center of a cupcake comes out clean about 17 minutes.
Step 8
Cool the cupcakes completely. Cut cores out of the center of each cupcake with a sharp paring knife. Discard cores.
Step 9
Bring cream to a simmer in a saucepan over low heat; stir in bittersweet chocolate until melted.
Step 9
Mix in 2 tablespoons butter and Irish whiskey until butter is melted; let mixture cool to room temperature. Filling will thicken as it cools.