Grease a 9x9 inch baking dish, then line it with parchment paper. Grease the inside of the parchment paper as well. Preheat the oven to 350 degrees.
In a small saucepan, bring the beer to a boil and reduce by about 2/3. Allow to cool while you work on the rest of the recipe.
Cream the butter and sugar in the bowl of a stand mixer or with a hand mixer. Next, beat in eggs one at a time, and finally add the beer.
Stir the flour and cocoa in by hand, until just blended. Transfer to the baking dish and bake for about 30 minutes, or until a tester inserted halfway into the middle of the pan comes out clean. Allow to cool completely.
Make the ganache. See link above.
Smooth ganache over the top. Allow 15-20 minutes to set up completely