Preheat oven to 350 degrees. Whisk cocoa powder, sugar, flour, baking soda, and salt together in a large bowl; set aside. In another, larger bowl combine stout, butter, and vanilla. Beat in eggs, then mix in sour cream.
Gradually mix dry ingredients into wet, adding no more than a cup of dry at a time. Grease muffin tin (if using paper liners, be sure to spray them with cooking spray or you'll have a hard time removing the paper!), then fill 3/4 full.
Bake 12 minutes, then rotate pan and bake 12-13 minutes longer. Cool a few minutes in the muffin tin until the cupcakes are cool enough to handle, then cool completely on a wire rack. Once completely cool, frost.
In a medium bowl with an electric hand mixer or the bowl of a stand mixer beat cream cheese until fluffy. Beat in heavy cream, then reduce to a low speed to beat in confectioner's sugar to desired consistency. After frosting, dust cupcakes with a little cocoa powder.