Preheat oven to 350°.
Add Guinness and butter to a saucepan and place over medium-low heat until the butter melts. Remove from heat. Add cocoa, sugar, cinnamon, and molasses. Whisk to combine.
Combine flour and baking soda in a large bowl, whisk to combine.
Add sour cream, eggs, and vanilla to a mixer, and mix well. Add the Guinness mixture and continue mixing.
Add the flour mixture a little at a time and mix well.
Butter two 9in cake pans and line with parchment.
Evenly distribute the batter to the cake pans.
Bake until risen. Be careful not to over bake. Around 25 mins.
Allow to cool completely.