Prepare the Cupcake Pan: Preheat the oven to 350°F and line a 12-count cupcake pan with cupcake liners; set the prepared pan aside.
Combine Dry Ingredients: In a large mixing bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt until evenly combined.
Mix Wet Ingredients: In a separate medium bowl, beat the eggs and granulated sugar until pale. Add the oil, Guinness stout, and vanilla extract, then beat until the mixture is fully combined.
Make the Batter: Pour the wet mixture into the bowl of dry ingredients and mix just until combined. Avoid overmixing, as this can make the cupcakes tougher.
Fill the Liners: Add a scant ¼ cup of batter to each cupcake liner, distributing the batter evenly among the 12 liners.
Bake the Cupcakes: Bake at 350°F for 17 to 20 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs but no wet batter.
Cool the Cupcakes: Let the cupcakes cool in the pan for several minutes, then transfer them to a wire rack and allow them to cool completely.
Prepare the Ganache: While the cupcakes bake, place the finely chopped bittersweet chocolate in a medium bowl. Heat the heavy cream and butter in a small saucepan or microwave-safe bowl just until the mixture begins to bubble around the edges. Rest the Chocolate: Pour the hot cream mixture over the chopped chocolate and leave it undisturbed for 5 minutes. Do not stir during this resting period.
Finish the Ganache: After 5 minutes, whisk the cream and chocolate together until smooth and rich. Stir in the Jameson Irish whiskey.
Cool the Ganache: Let the ganache stand at room temperature for 15 to 30 minutes before using it to fill the cupcakes.
Core the Cupcakes: Once the cupcakes are completely cool, remove about 1 to 2 tablespoons of cake from the center of each cupcake using a cupcake corer. A paring knife can also be used carefully, taking care not to cut too deeply.
Fill the Cupcakes: Spoon or add the cooled ganache into the hollow center of each cupcake.
Make the Frosting: In a large bowl or stand mixer fitted with a paddle attachment, beat the softened butter together with 1 cup of confectioners’ sugar until combined.
Finish the Buttercream: Add the remaining 1 cup of confectioners’ sugar, Baileys Irish Cream, and salt. Beat on high speed until the frosting becomes light and fluffy.
Pipe the Frosting: Pipe the Baileys buttercream over the filled cupcakes using a piping bag and your preferred piping tip.
Decorate the Cupcakes: Add sprinkles over the frosting if desired, then serve. Store the finished cupcakes in an airtight container for up to 3 days. Alternatively, the cupcakes may be baked up to 1 day ahead and filled and frosted later.
