Guinness Chocolate Cupcakes with Baileys Buttercream

Preheat oven to 350°F (175°C). Line two muffin tins with cupcake liners.
In a medium bowl, whisk together flour, cocoa powder, instant espresso (if using), baking soda, baking powder, and salt.
In a large mixing bowl, beat the softened butter with both the brown sugar and granulated sugar using an electric mixer on medium-high speed until light and fluffy, about 3–4 minutes.
Reduce mixer speed to low. Beat in eggs, one at a time, scraping down sides as needed, then add vanilla extract and sour cream. Mix until smooth.
Add the dry ingredients in three additions, alternating with the Guinness. Start and end with dry ingredients. Mix only until just incorporated—do not overmix.
Divide batter among cupcake liners, filling each about 2/3 full.
Bake for 16–18 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Let cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.
For the Baileys Buttercream: In a large bowl, beat butter until smooth and fluffy. Gradually add powdered sugar, vanilla extract, and salt, beating until smooth. Add Baileys and beat until creamy. Adjust consistency by adding more Baileys (if too thick) or more sugar (if too thin).
Frost the cooled cupcakes using a piping bag or spatula. Garnish with chocolate shavings or sprinkles if desired.