In a small pot add Guinness stout and butter. Simmer over medium heat until butter melts.
Preheat oven to 350F. Prepare cupcake tin with cupcake liners.
Add cocoa powder and whisk to combine. Set aside to cool.
In a medium bowl add flour, sugar, baking powder, and sugars. Whisk to combine.
In the bowl of a stand mixer (or large bowl if using a hand mixer), add sour cream, egg, and vanilla. Mix on low for about 30 seconds to combine.
Add Guinness mixture to sour cream mixture. Mix on low until combined. About 30 seconds.
Add dry ingredients to liquids. Mix on low making sure all flour is combined. About 30 seconds.
Bake for about 20 minutes. Cupcakes are done when middle springs back when pressed, or toothpick comes out of center clean.
Cool in cupcake tin for about 5 minutes, until cupcakes have cooled enough to handle. Remove from cupcake tin and allow to fully cool on wire rack before topping with buttercream.