Preheat your oven to 350°F (175°C) and prepare a cake pan.
In a saucepan, combine the Guinness and butter over medium heat until melted.
Whisk in the sugar and cocoa powder until smooth.
In a large mixing bowl, combine flour, baking soda, and salt.
In a separate bowl, beat the eggs, sour cream, and vanilla extract together until well combined.
Gradually combine the wet and dry ingredients, mixing until just combined.
Pour the batter into the prepared cake pan and bake for 45-50 minutes, or until a toothpick comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
While the cake cools, beat together the cream cheese and powdered sugar until smooth.
Once the cake is cool, spread the cream cheese frosting evenly on top.