Baileys Chocolate Ganache Frosting (make in advance):
Place chopped chocolate in a large heatproof bowl.
In a medium saucepan, combine heavy cream, Baileys, and butter. Cook over med heat, stirring often until it just barely starts to simmer.
Pour hot cream over chopped chocolate and cover bowl with plastic wrap. Let sit 5 minutes.
Stir gently with a spatula until completely smooth. If needed, place over a double boiler and stir until all chocolate has melted.
Place plastic wrap directly on top of chocolate ganache. Allow to thicken and set overnight.*
Chocolate Guinness Cake:
Preheat oven to 350F, grease three 6" round baking pans and dust with cocoa powder. Line bottoms with parchment.
Place flour, sugars, cocoas, baking powder, baking soda, and salt into the bowl of a stand mixer fitted with a paddle attachment. Stir to combine.
Place Guinness into a small saucepan and heat until just simmering.
In a medium bowl whisk buttermilk, eggs, oil, and vanilla. Temper this mixture with the heated Guinness -- pour hot Guinness into the bowl very slowly while whisking egg mixture quickly.
Add wet ingredients to dry and mix on medium for 2-3 mins. Batter will be very thin.
Pour evenly into prepared pans. I used a kitchen scale to ensure the batter is evenly distributed.
Bake until a cake tester comes out mostly clean. A total of 30-35mins.
Cool 10 minutes in the pans then turn out onto a wire rack to cool completely.
Baileys Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.**
Place bowl over a hot water bath on the stove and whisk constantly until the mixture is hot (160F) and no longer grainy to the touch (approx. 3mins).
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.
Add Baileys and vanilla and whip until smooth.***
Cut each layer of cake in half horizontally.
Place one layer of cake on a cake stand or serving dish. Top with about 1/2 cup buttercream, spread evenly. Repeat with remaining layers alternating buttercream with ganache. Frost and smooth the outside with a thin crumb coat. Chill for at least 20mins until firm and set.****
Frost the chilled cake with the remaining ganache. It will start to set quicjkly against the cold cake.
To smooth the top and sides of the cake, use a metal bench scraper and offset spatula that have been run under hot water (wipe them off before use). Keep heating the scraper/spatula and repeat the process until the sides and top are smooth.
* You can speed up the thickening of the ganache by placing it in the fridge, but you need to stir it often.** Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk are completely grease free or your meringue won't stiffen.*** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.**** If the ganache is too thin or soft and the cake layers start to slide around a bit during assembly, you can place 3-4 bamboo skewers into the top of the cake as support. You'll need to trim the skewers to the proper height of the cake.