Preheat the oven to 350°F. Grease an 8-inch spring-form pan with butter and line the bottom with a round of parchment paper.
In a large saucepan place the beer and butter. Cook over medium-high heat until the butter has melted. Add the cocoa powder and sugars and whisk together. Take off heat and allow to come to room temperature.
In another bowl beat together the sour cream, eggs, and vanilla extract until thoroughly combined. Add to the butter-beer mixture and whisk together.
In another bowl whisk together the flour, baking soda, and salt. Add to the beer-butter mixture and whisk together until it just comes together. Pour into the prepared pan, banging the pan on the countertop to remove any air bubbles. Bake for 50-60 minutes. Allow to cool on a wire rack.
Run a knife around the edge to separate the cake from the pan and remove the pan’s side.
Spread frosting on the cooled cake and sprinkle with the crushed pretzels.