Preheat your oven to 350°F (177°C) and prepare a muffin pan by lining it with cupcake liners.

In a medium saucepan over low heat, melt the unsalted butter together with the Guinness until smooth. Stir in the unsweetened cocoa powder and let cool slightly.

In a large mixing bowl, whisk together all-purpose flour, granulated sugar, baking soda, and salt until well combined.

Mix the large egg and sour cream together, then gradually add the dry ingredients alternately with the chocolate mixture until just combined.

Fill each cupcake liner about two-thirds full with the prepared batter.

Bake for about 20 minutes, or until a toothpick inserted into the center comes out clean.

Allow them to cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.

For the ganache, heat the heavy cream until it begins to simmer, pour over chocolate chips, and let sit before stirring until smooth.

Cool the ganache to room temperature and then pipe it into each cupcake.

Whip the heavy cream until soft peaks form, gradually adding powdered sugar, then folding in the Irish cream.

Pipe the frosting onto each cupcake and add sprinkles or chocolate shavings if desired.