In a small saucepan, melt butter and whisk together with Guinness, vanilla extract and cocoa. Remove from heat.
While the Guinness mixture is cooling, grease and line 2 eight inch cake pans with parchment paper. Next, whisk together the dry ingredients (sugar, flour, baking soda). Pour the Guinness mixture onto the dry ingredients, then whisk in the 2 eggs.
When the batter is shiny and smooth, pour evenly into two prepared cake pans.
Bake for 30-35 minutes, or until a skewer comes out clean.
Meanwhile, make the buttercream by whipping together the softened butter and sugar in a standing mixer, then adding just enough Bailey’s to get it loose and fluffy. They key to white frosting is to whip it a long time—the longer you whip it, the whiter it will become. I whipped this for 5-10 minutes. I only needed 3 tablespoons Bailey’s.
Once the cakes are done baking, cool completely.
After they’ve cooled, assemble the cake (leveling the layers with a serrated knife, if needed). First, add the frosting for the middle layer. I used about 1/3 of the buttercream.
Top with second layer. Wiggle them around until they line up just right. Next we’ll do a crumb coat. This is optional, but highly recommended, so you don’t get brown crumbs in your white frosting. Here’s how it works. Coat top and sides with another 1/3 of the frosting mixture.
Spread it all over, nice and thin. Refrigerate to set it. At this point, you can refrigerate the cake overnight.
Once the crumb coat is firm to the touch, add the final 1/3 of the frosting to the cake—top first, then sides. Spread it around evenly. When you’re done, slice and serve with an extra cod Guinness.