Preheat your oven to 350°F and grease a 9-inch springform pan.
Pour 1 cup Guinness stout and 1 cup unsalted butter into a saucepan, then heat over medium until the butter melts completely.
Whisk in ¾ cup unsweetened cocoa powder until the mixture is smooth and free of lumps.
Whisk in ¾ cup unsweetened cocoa powder until the mixture is smooth and free of lumps. Tip: Use a whisk to blend the dry ingredients—this prevents overmixing later and ensures a tender crumb.
Pour the warm Guinness mixture into the dry ingredients and stir just until combined.
Beat in 2 large eggs and ⅔ cup sour cream until the batter is uniform and glossy. Tip: Scrape the bowl’s sides with a spatula to incorporate every bit of batter for even baking.
Transfer the batter to the prepared pan and bake at 350°F for 45–50 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cake in the pan on a wire rack for 15 minutes, then release the springform and let it cool completely.
For the frosting, beat 8 oz softened cream cheese with 1½ cups powdered sugar until fluffy. Gradually add ½ cup heavy cream, whipping until the frosting holds soft peaks. Tip: Chill the frosting for 10 minutes before spreading—it’ll be easier to handle and less likely to slide off the cake. Frost the cooled cake evenly, swirling the top with a knife for a rustic look.