Preheat oven to 350F. Grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, baking powder, and salt and set aside.
Beat butter until smooth. Add sugar and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Mix until just combined
Spread batter evenly into prepared pan and smooth the top with a spatula.
Bake for 30mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Place chopped chocolate and cream into a microwave safe bowl, stir. Microwave on high for 30 seconds, stir. Again, microwave on high for 30 seconds, stir until smooth. Cool completely until thickened, ideally overnight.
Coffee Baileys 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 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 espresso powder. Whip until smooth.**
Place one layer of cake on a cake stand or serving plate. Spread a layer of ganache and then top with buttercream. Repeat with remaining layers and crumb coat the cake. Chill for 20mins.
* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is 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.