Place a rack in the center of the oven and preheat to 350°F (175°C). Grease a 9-inch springform pan with cooking spray, then dust it with cocoa powder and tap out the excess.

In a medium saucepan over medium-high heat, bring the stout to a boil. Cook while stirring occasionally until the liquid reduces to about 1 cup, around 15 to 20 minutes. Allow it to cool to room temperature.

In a large bowl, beat the eggs and granulated sugar using a handheld mixer on medium-high speed until the mixture becomes pale and fluffy, about 2 minutes. Add the sour cream, vegetable oil, and vanilla extract, then beat until fully combined.

Pour the cooled Guinness into the egg mixture and whisk until smooth and evenly incorporated.

In a separate medium bowl, whisk together the flour, baking soda, cocoa powder, and salt. Add this dry mixture to the egg mixture and gently fold until just combined, making sure not to overmix. Pour the batter into the prepared pan.

Bake the cake for 50 to 60 minutes, or until a tester inserted in the center comes out mostly clean. It should not come out completely clean to keep the cake moist. Place the pan on a wire rack and let it cool.

Frosting

In a large bowl, beat the softened cream cheese using a handheld mixer on medium-high speed until light and fluffy, about 2 minutes. Add the confectioners’ sugar and vanilla, mixing gently with a spatula until incorporated. Continue beating with the mixer on medium-high speed until the frosting becomes light and fluffy.

Spoon the frosting onto the center of the cooled cake. Use the back of a spoon or an offset spatula to spread it across the top as desired.