Preheat oven to 350°F (175°C). Grease and line a 9-inch springform pan or two 8-inch round cake pans.,In a saucepan over medium heat, combine Guinness and butter. Heat until butter melts, then whisk in cocoa powder and sugar until smooth. Remove from heat and let cool slightly.,In a separate bowl, whisk together sour cream, eggs, and vanilla extract. Stir into the chocolate mixture until combined.,Sift in flour, baking soda, and salt. Gently mix until no flour streaks remain. Do not overmix.,Pour batter into the prepared pan(s) and bake for 45–50 minutes (springform) or 30–35 minutes (round pans), until a toothpick inserted in the center comes out with moist crumbs.,Cool completely before frosting.,For ganache: Heat cream just to a simmer, pour over chopped chocolate, let sit 1 minute, then stir until smooth. Cool slightly before pouring over cake.,For cream cheese frosting: Beat cream cheese and butter until smooth. Gradually add powdered sugar and vanilla until fluffy. Spread over cooled cake.