Preheat oven to 350 degrees.
In a large mixing bowl, mix the cocoa, sugar, flour, baking soda and salt until thoroughly mixed.
In a medium-sized bowl, combine the stout beer, melted butter and vanilla. Beat in eggs, adding one at a time. Mix in sour cream and mix thoroughly until smooth.
Gradually add the dry ingredients to the wet ingredients.
Lightly grease cupcake or muffin tins. Pour batter into muffin tins, filling to 3/4 full. Bake for 12 minutes, then rotate the pan in the oven, and bake an additional 12 minutes.
Remove from oven and let cool before taking them out of the muffin tin.