Preheat the oven to 350°F. In a small saucepan, heat the butter until just melted, then whisk together with Guinness, vanilla extract, and cocoa. While the Guinness mixture is cooling, grease and line the bottoms of two 8-inch cake pans with rounds of parchment paper. Whisk together the sugar, flour, and baking soda in a large bowl. Pour the Guinness mixture onto the dry ingredients, and then whisk in the 2 eggs. When the batter is shiny and smooth, pour it into two prepared cake pans. Lick the whisk when no one is looking. Bake for 30 to 35 minutes, or until a skew
For the buttercream:
In a stand mixer, whip the softened butter until fluffy. Add the powdered sugar on low speed, then increase to medium-high, and drizzle in just enough Baileys to get the buttercream loose and fluffy. The key to making whiter frosting is to whip it 5 to 10 minutes, scraping occasionally.
To assemble the cake:
Run a knife around the edge of the cake pan to loosen and turn out cakes. Level the layers with a serrated knife, if needed. Spread about a third of the buttercream on the bottom cake layer. Top with the second layer. Wiggle them around until they line up just right.
Thinly spread another third of the frosting mixture over the top and sides of the cake to make a crumb coat. This will seal in the crumbs so chocolate flecks don't ruin the white frosting. Refrigerate to set—about 30 minutes or overnight if desired.
Once the crumb coat is firm to the touch, add the final t