Preheat oven to 350°F (175°C). Grease a 10-inch bundt pan thoroughly with butter or non-stick spray, then dust with flour or cocoa powder to prevent sticking.

In a large bowl, sift together the flour, cocoa powder, and baking soda. Whisk in the sugar until evenly mixed.

Melt the butter gently in a small saucepan or microwave-safe bowl. Pour in the Guinness stout and heat until just warm, about 1-2 minutes.

In a separate bowl, whisk the eggs, sour cream, and vanilla extract until smooth. Slowly pour the warm Guinness and butter mixture into the eggs, whisking constantly to prevent scrambling.

Pour the wet mixture into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Do not overmix.

Pour the batter evenly into the prepared bundt pan. Shake gently to level the batter.

Bake for 45-50 minutes, or until a toothpick inserted near the center comes out clean or with a few moist crumbs.

Let the cake cool in the pan on a wire rack for 15 minutes. Then invert onto the rack to cool completely before glazing.

To make the stout glaze, whisk together powdered sugar, cocoa powder, melted butter, and 2 tablespoons Guinness stout in a small bowl. Add more stout a teaspoon at a time until glaze is pourable but thick enough to coat the back of a spoon.

Drizzle the stout glaze over the cooled cake in a slow, steady stream. Let set for about 10 minutes before slicing.