Preheat oven to 350 degrees F. Grease three (or two if that's all you have) 6-inch round cake pans and line the bottoms with parchment paper. (See Notes)
In the bowl of a stand mixer fitted with a paddle attachment, beat butter and sugars on high speed until pale and creamy, about 3-4 minutes. Scrape down the bowl as needed.
Add the vanilla and then the eggs and yolks one at a time, mixing after each.
In a separate bowl, combine the cake flour, baking powder, baking soda, salt and nutmeg. Add to the mixer in 3 additions, alternating with the buttermilk, scraping down the bowl as needed. Mix in the bourbon.
Divide the batter evenly between the prepared cake pans and bake until edges are golden, and the tops bounce back when gently pressed, about 25 minutes.
Let cool in the pans for 15 minutes, then carefully transfer to a wire rack to cool completely. (If using only two pans, you can bake in 2 batches, washing a pan before baking the third layer).
Wrap in plastic and chill in the fridge until ready to assemble the cake.