Preheat oven to 350.
Add beer and butter to a saucepan and heat until butter is melted. Remove from heat and thoroughly whisk in the cocoa powder. Set aside to cool.
In a mixing bowl, combine eggs and sour cream. Add in the beer mixture, thoroughly mixing with an electric mixer.
In a separate bowl, whisk together the flour, sugar, baking soda, and salt.
Pour the beer mixture into the dry ingredients, mixing again with your electric mixer.
Pour the batter into two 9" cake pans and bake for about 30 minutes.
Make your Bailey's buttercream icing by combining the butter, powdered sugar, and Bailey's (start with just one tablespoon, adding additional tablespoons as needed so your icing isn't too thin).
Use the electric mixer to combine thoroughly, blending for about 10 minutes.
Wait until your cake has cooled completely before adding the icing.