Preheat the oven to 350℉. Grease and flour two 9” cake molds.
Melt the butter in a saucepan on medium heat. Add the beer and cocoa and whisk until smooth. Remove from heat and leave to cool for 15 minutes.
In a large bowl, combine the flour, sugar, baking powder and salt. Set aside.
In another large bowl, add the eggs, sour cream and vanilla and whisk to blend well.
Pour the chocolate mixture into the egg mixture and using an electric mixer, mix on low for 30 seconds.
Gradually add the flour mixture and combine for 1 minute. Scrape down the sides well.
Distribute the mixture in the cake molds, place in the oven and bake for 35 to 40 minutes or until a toothpick inserted in the centre comes out clean. Remove from the oven, place on a rack and let sit for 15 minutes. Unmold and let cool completely before icing.
For the icing: put the butter in a large bowl and mix on medium for 4 to 5 minutes. Add the icing sugar one cup at a time, using the mixer to combine it into the mixture between each cup.
Add the beer, vanilla and salt and mix for 2 minutes until well combined. Give the sides a good scrape!
Place one layer of the cake on your favourite plate and spread a third of the icing on top. Place the other layer of the cake on top and ice with the rest of the icing. Decorate with cocoa and serve!