With the rack in the middle position, preheat the oven to 350°F (180°C). Line two 8-inch (20 cm) springform pans with parchment paper. Butter and flour the sides.

In a bowl, combine the flour and baking soda. Set aside.

In a saucepan over medium heat, melt the butter with the beer and cocoa powder, stirring with a whisk. Let cool.

In a large bowl, combine the brown sugar, sugar and salt. With a whisk, add the lukewarm butter mixture alternately with the dry ingredients and eggs.

Add the sour cream and stir until the mixture is smooth.