Preheat the oven to 350 degrees F. Line standard cupcake pans with 18 cupcake liners.

In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar until light and smooth. Add the eggs, 1 at a time, beating until each is smooth, scraping down the sides of the bowl if needed. Add the vanilla. Sift the cocoa, flour, baking powder, soda, and salt in a large bowl. In the creamed bowl, alternate the leftover ingredients beginning and ending with dry ingredients. Stir in the sauerkraut. Fill the cupcake liners 2/3 full. Bake until the cupcake springs back when touched, about 35 minutes. Transfer the pans to wire racks to cool completely before removing the cupcakes.

To assemble: Frost the cooled cupcakes with Beer Frosting.

In the bowl of an electric mixer fitted with the paddle attachment, mix the softened butter with half the confectioners' sugar. Add the cocoa, salt, and roughly 1/3 of the beer. Beat until smooth. Alternate adding the remaining sugar with the remaining beer and beat until fluffy.