Preheat Oven:
Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line with parchment paper.
Prepare Cake Batter:
In a medium saucepan over medium heat, combine Guinness and butter. Heat until butter melts. Remove from heat and whisk in cocoa powder until smooth.
In a large bowl, mix sugar, eggs, and sour cream until well combined. Stir in the Guinness-butter-cocoa mixture.
In another bowl, whisk together flour, baking soda, and salt. Gradually fold the dry ingredients into the wet mixture until just combined. Stir in vanilla extract.
Bake:
Divide batter evenly between prepared pans. Bake for 45–50 minutes, or until a toothpick inserted in the center comes out clean. Cool cakes in pans for 10 minutes, then transfer to wire racks to cool completely.
Make Frosting:
In a large bowl, beat softened butter until creamy. Gradually add powdered sugar, beating until smooth.
Add Irish cream, vanilla, and a pinch of salt. Beat until fluffy. Adjust consistency with more powdered sugar if too thin or more Irish cream if too thick.
Assemble Cake:
Place one cake layer on a serving plate. Spread a generous layer of frosting on top.
Place the second layer on top and frost the top and sides of the cake evenly.