Preheat oven to 350 degrees. Generously butter 2 9-inch cake pans.
In a large bowl, beat together the butter, brown sugar, and sugar until soft and fluffy, about 2-3 minutes. Whip in the eggs, one at a time, until incorporated, then the vanilla extract and Greek yogurt. Add the flour, cocoa powder, baking powder, baking soda, and salt. With the mixer running, stream in the buttermilk. Mix until just combined.
Divide the batter among the two prepared cake pans. Bake for 35-40 minutes until an inserted toothpick emerges with just a few moist crumbs. Let cool 10 minutes in the pans, then run a knife around the edges and flip the cakes onto a wire rack. Cool completely.
When the cakes are cool, make the buttercream. In a large bowl, beat the butter, bourbon, and vanilla on low speed until combined. Add the powdered sugar 1 cup at a time, mixing to incorporate. Turn the mixer to medium high and add 1-2 tablespoons of heavy cream. Beat on high speed until very soft and fluffy, about 2-3 minutes.
If desired, carefully slice the cake layers in half to create 4 layers. You can also leave it as 2 if you wish. Lay one layer on the bottom of a flat plate or cake stand. Frost with some of the buttercream. Lay a second layer on top; frost that one. Repeat with the remaining 2 cake layers. Spread the remaining buttercream all over the cake. Slice and serve!