Prep the pan and oven. Heat the oven to 350°F (175°C). Grease a 9-inch round springform pan or a 9x13-inch pan. Line the bottom with parchment and lightly flour the sides.

Warm the Guinness and butter. In a medium saucepan over low heat, combine the Guinness and butter. Warm until the butter melts, then remove from heat. Whisk in the cocoa powder and sugar until smooth.

Mix the wet ingredients. In a separate bowl, whisk together sour cream, eggs, and vanilla until well combined and silky.

Combine wet with chocolate base. Slowly whisk the sour cream mixture into the warm chocolate mixture. It should look glossy and uniform.

Add the dry ingredients. In another bowl, whisk flour, baking soda, and salt. Sift if lumpy. Gently fold this into the batter in two additions. Mix just until no dry streaks remain to keep the crumb tender.

Pour and bake. Pour the batter into the prepared pan. Bake for 45–55 minutes for a 9-inch round, or 30–35 minutes for a 9x13-inch pan. The cake is done when a toothpick comes out with a few moist crumbs.

Cool completely. Let the cake cool in the pan for 15 minutes, then transfer to a wire rack. Cool fully before frosting so the frosting stays smooth and doesn’t melt.

Make the frosting. Beat cream cheese and butter together until fluffy and smooth, about 2–3 minutes. Add powdered sugar gradually, then vanilla and a pinch of salt. Add 1–2 tablespoons milk or cream to reach a spreadable consistency.

Frost generously. Spread a thick layer of frosting over the cooled cake. Aim for soft swoops rather than overworking it—this keeps it airy and elegant.

Rest, slice, and serve. For the cleanest slices, chill the frosted cake for 20–30 minutes, then cut with a warm, sharp knife. Serve slightly cool or at room temperature.