Preheat the oven to 325 degrees F. Line a cupcake or muffin pan
with 24 regular-size cupcake liners.
Place the stout and butter in a medium-size saucepan and whisk together on medium heat until the butter is melted. Remove from the heat. Sift
the cocoa powder
into a medium-size bowl and add the sugar. Slowly whisk
into the stout mixture. In a small bowl, combine the sour cream, eggs, and vanilla and lightly whisk until smooth. Add this mixture to the saucepan and whisk thoroughly (it may appear lumpy). Sift the flour and baking soda together in another small bowl and then add it to the saucepan
, mixing a final time until the color is even.
Fill the cupcake liners three-quarters full with batter
and bake until the cakes
spring back after touching, about 27 minutes. Cool the cupcakes in their pan for 5 minutes before transferring to a cooling rack
To assemble: Pipe the frosting onto cooled cupcakes using a large plain tip. Dust
with cocoa powder if preferred.