Preheat the oven to 350°F/175°C. Grease 6 mini pans or an 8 or 9-inch springform pan with butter or Crisco and line the bottoms with rounds of parchment paper.
In a small heavy-bottomed saucepan, heat the Guinness and butter until butter is melted.
Remove from the heat; whisk in sugar and cocoa until blended.
Combine the eggs, sour cream and vanilla and mix by hand until well blended.
Whish the egg mixture into the saucepan and stir until combined.
Blend flour and baking soda; whisk into beer mixture until smooth.
Pour batter into prepared pan(s) and bake for 20 minutes in small pans. If using 8 or 9-inch springform pans bake up to 60 minutes.
Remove from oven and cool on wire rack for 20 minutes. Remove from Springform pans and allow them to sit until thoroughly cooled.