How to Make It
Heat oven to 350° F. Butter a 12-cup Bundt pan and dust with the cocoa powder, tapping out the excess. In a medium bowl, whisk together the flour, baking soda, and salt.
In a small saucepan, combine the butter and stout. Cook over medium heat, stirring, until the butter is melted. Remove from heat, add 8 ounces of the chocolate, and whisk until smooth.
Using an electric mixer, beat the eggs and sugars on medium-high until fluffy. Beat in the chocolate mixture and sour cream. Reduce speed to low and gradually mix in the flour mixture until just combined (do not overmix).
Pour into the prepared pan and bake until a toothpick inserted in the center comes out with a few moist crumbs attached, 45 to 55 minutes. Let cool 30 minutes in the pan, then invert onto a rack to cool completely.
In a small saucepan, bring the heavy cream just to a boil. Remove from heat, add the remaining 4 ounces of chocolate, and let sit 5 minutes. Whisk until smooth. Set the cooling rack with the cake over a baking sheet. Drizzle the cake with the glaze and let set before serving.