In a mixing bowl, combine the flour, sugar, cocoa powder, baking soda, and salt. Whisk them together until they’re evenly incorporated. This creates a solid foundation for our cupcakes.

In a separate bowl, beat together the soft butter and sugar until fluffy. Then, add the eggs one at a time, mixing well after each addition. Gradually stir in the sour cream, Guinness, vanilla extract, and espresso powder. The liquid ingredients should be well combined for a rich batter.

Slowly add the dry ingredients to the bowl with the wet mixture. Stir with a spatula until just combined. Beware of over-mixing. You want a smooth, slightly thick batter without any lumps.

Preheat the oven to 350°F (175°C). Line your muffin tin with cupcake liners. Fill each liner about two-thirds full with batter. Bake for about 18-20 minutes or until a toothpick comes out clean. Let them cool on a wire rack.

While your cupcakes cool down, it’s time to make the ganache. Heat the heavy cream until it’s steaming, but not boiling. Pour it over the chopped bittersweet chocolate in a bowl, letting it sit for a minute. Add the whiskey and butter, and stir until smooth. Allow it to cool and thicken slightly.

Once the cupcakes are cool, use a small knife or a cupcake corer to remove the center of each cupcake. Spoon in the whiskey ganache filling, filling until just above the top.

Beat the softened butter in a large bowl until creamy. Gradually add in the powdered sugar, mixing until well blended. Then, pour in the Baileys Irish Cream and vanilla extract. Beat until the frosting is fluffy and smooth.

Using a decorating bag or a knife, spread the Baileys frosting over the tops of the filled cupcakes. Swirl it for a professional touch.

For that bakery-style finish, you might want to drizzle some ganache over the frosted cupcakes. It's an extra step but adds visual flair!

These cupcakes are best served fresh but can be stored for a couple of days.