Arrange your oven rack to the middle position. Preheat your oven to 350°F and line a 12-count muffin pan with cupcake liners.
In a medium bowl, whisk the flour, sifted cocoa powder, baking powder, baking soda, and salt.
In a large bowl, whisk the vegetable oil, sugar, eggs, vanilla extract, melted chocolate, Guinness beer, and sour cream until smooth. Whisk in the dry ingredients until the batter is smooth with no pockets of flour remaining.
Divide the batter evenly between the cupcake liners. Each one should be filled about ⅔ of the way up.
Bake in the preheated oven for 15-17 minutes, or until a toothpick inserted into the center comes out clean.
Let the cupcakes cool slightly in the pan before removing, then cool completely before adding the Guinness chocolate ganache.