For the cake:
- 1 cup cocoa powder, plus extra for the pan
- 1 cup (2 sticks) unsalted butter, softened
- 2 ½ cups granulated sugar
- 4 large eggs
- 2 ½ cups all-purpose flour
- ½ teaspoon baking powder
- ½ teaspoon baking soda
- ½ teaspoon kosher salt
- ⅛ teaspoon ground nutmeg
- 1 tablespoon vanilla extract
- ½ cup half-and-half
- ½ cup Guinness, room temperature
For the glaze:
- 2 cups powdered sugar
- 3 tablespoons cocoa powder
- 6 tablespoons Guinness
- 2 tablespoons half-and-half or heavy cream
- ½ teaspoon vanilla extract
- Pinch of kosher salt
Heat the oven to 325 F. Coat a 12-cup Bundt pan very well with cooking spray. Sprinkle in a bit of cocoa powder, turning to coat evening, then overturning to discard any excess.
In a large bowl, use an electric mixer to beat together the butter and sugar. Add the eggs, one at a time, and beat until smooth. Set aside.
In a large bowl, whisk together the 1 cup of cocoa powder, the flour, baking powder, baking soda, salt, and nutmeg. In a 2-cup liquid measuring cup, stir together the vanilla, half-and-half and Guinness.
With the mixer running on low, alternate adding the liquid and dry ingredients to the sugar-butter mixture, mixing just until everything is mixed and smooth. Transfer to the prepared Bundt pan, tapping it gently on the counter to release air bubbles. Smooth the top, then bake for 1 hour 10 minutes, or until a toothpick inserted at the center comes out clean.
Meanwhile, prepare the glaze. In a medium bowl, whisk together all ingredients until smooth. Set aside.
When the cake is done, let it cool in the pan for 25 minutes. Set a wire cooling rack over the pan, then invert it so the cake stands on the rack. For ease of cleanup, set the rack over a rimmed baking sheet or a sheet of kitchen parchment. Use a toothpick to poke deep holes all over the top of the cake.
Spoon glaze over the top of the cake and wait until it seeps into the holes. Spoon more glaze into the holes, then wait 10 minutes. Spoon the remaining glaze all over the top so it covers the holes and drizzles down the sides.