How to Make It
Make the cake: Heat oven to 350° F. Butter an 8-inch square baking pan. Line with parchment, leaving an overhang on 2 sides. Butter the parchment.
Whisk together the flour, cocoa, baking powder, and salt in a medium bowl; set aside. In a separate bowl, whisk together the butter, sugar, eggs, beer, and vanilla until smooth. Add the flour mixture and mix until just combined.
Spread the batter in the prepared pan and bake until a toothpick inserted in the center comes out clean, 20 to 24 minutes. Let cool completely in the pan. Holding the paper overhang, lift the cake out of the pan and transfer to a cutting board.
Make the glaze: Bring the beer and cream to a boil in a small pot. Remove from heat, add the chocolate, and let stand for 5 minutes. Whisk until smooth. Cool slightly, then pour over the cake.
Let the glaze set, about 10 minutes. Cut the cake into 9 squares.