Preheat oven to 350°F (175°C) and butter two 9-inch round cake pans lined with parchment.

In a saucepan, melt the butter and Guinness over medium heat. Off the heat, whisk in the cocoa until smooth and let cool 10 minutes.

In a large bowl, whisk the sugar, eggs, sour cream, and vanilla until thick and pale. Whisk in the cooled stout mixture.

Whisk the flour, baking soda, and salt in a small bowl, then fold into the wet ingredients in two additions just until no dry streaks remain.

Divide the batter between the pans and bake 30 to 35 minutes, until a toothpick comes out with moist crumbs.

Cool in pans 10 minutes, turn out, peel parchment, and cool completely on a rack before slicing.