Arrange a rack in center of oven; preheat to 350°. Grease a 9" springform pan with cooking spray. Dust with cocoa powder, tapping out excess.

In a medium saucepan over medium-high heat, bring stout to a boil. Cook, stirring occasionally, until reduced to about 1 cup, 15 to 20 minutes. Let cool until room temperature.

Meanwhile, in a large bowl, using a handheld mixer on medium-high speed, beat eggs and granulated sugar until pale and fluffy, about 2 minutes. Add sour cream, oil, and vanilla and beat until combined.

Add Guinness to egg mixture and whisk until well incorporated and smooth.

In a medium bowl, whisk flour, baking soda, cocoa powder, and salt. Add to bowl with egg mixture and fold until just combined; do not overmix. Pour batter into prepared pan.

Bake cake until a tester inserted into the center comes out mostly clean, 50 to 60 minutes. (It’s important for it to not come out completely clean, as you don’t want this to be a dry cake.) Transfer pan to a wire rack and let cool.

In a large bowl, using handheld mixer on medium-high speed, beat cream cheese until light and fluffy, about 2 minutes. Add confectioners’ sugar and vanilla and gently mix with a spatula until just incorporated. Using handheld mixer on medium-high speed, continue to beat until frosting is very light and fluffy.

Spread frosting in the center of top of cake. This ensures you can control how much you want it spread. Using the back of a spoon or an offset spatula, spread frosting as desired.