Preheat oven to 350 degrees F.
Spray a standard bundt pan with cooking spray and set aside.
Whisk together the flour, sugar, cocoa powder, baking soda, baking powder and salt.
With a hand or stand mixer, beat together the buttermilk, Guinness, oil, eggs and vanilla on low setting.
Add the flour mixture and beat until combined, then beat the cake batter for 4 minutes on medium speed.
Pour the batter into the prepared bundt pan.
Bake the cake for 45 - 50 minutes, or until cake tester comes out clean and the top of the cake springs back when lightly touched with your finger.
Allow to cool in the pan for about 15 minutes, remove from pan and allow cake to cool on rack.