Preheat oven to 350 degrees F and line and grease two 9 or 8-inch round cake pans with parchment paper and cooking spray and set aside. Click for the photo instructions for lining round pans with parchment paper.
Combine the sugar, flour, cocoa powder, baking soda, baking powder, and salt in a large bowl or stand mixer fitted with a whisk attachment and mix together.
Beat in the eggs, buttermilk, Guinness, oil, and vanilla and beat on medium speed for 2 minutes, the batter is going to be very thin.
Evenly divide the batter between the two pans and bake the cakes for 40-45 minutes until toothpick comes clean from center.
Cool in pans for at least 10 minutes and loosen from sides of pan before transferring to a cooling rack to cool completely. I like to use Bake Even Strips for a nice and evenly baked cake, but if your cakes dome, you'll need to gently slice the tops of the cake off so they are level before assembling.
While the cakes are cooling, prepare the ganache by chopping the chocolate and placing it in a medium bowl. Heat the heavy cream in a small saucepan just until simmering around the edges then pour over the chocolate and let sit for 4 minutes. Once the time has elapsed, whisk until a rich chocolate ganache forms.
Allow the ganache to cool and thicken a bit before spreading on the top of one of the layers of cake about 1/4-inch from the edge of the cake, and set aside.
Prepare the Irish Cream Frosting, by creaming the butter in a large bowl or stand mixer fitted with a paddle attachment. Add in the powdered sugar and Irish Cream liqueur a little at a time alternating between each. If frosting seems too stiff, add a little more liqueur.
Pipe a line of the frosting around the outer edge of the ganache to seal it in before adding the top layer. Then apply a thin crumb coat of the frosting to the entire cake and place in the fridge for 20 minutes or the freezer for 5 minutes.
Use the remaining frosting to finish frosting the cake. Add chocolate sprinkles or other sprinkles as desired.