First, make the bourbon buttermilk bundt cake. Position a rack in the center of the oven and preheat the oven to 350°F. Line a half sheet pan with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. In a liquid measuring cup, whisk together the buttermilk, whiskey, and vanilla. In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugars. Beat on medium-high until light, fluffy, and doubled in volume, 3 to 5 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary. Reduce the mixer to low and add the eggs one at a time, adding the next egg only after the previous one has been fully incorporated, scraping down the bottom and sides of the bowl after each addition.
With the mixer still on low, add the the dry ingredients in three parts, alternating with the wet ingredients in two parts, beginning and ending with the dry ingredients. Beat until just combined, then scrape down the bottom and sides of the bowl once more, then beat on low for an additional 30 seconds.
Prep and fill your bundt pan. Generously spray a 10-cup capacity bundt pan with cooking spray. Immediately pour the batter into the prepared bundt pan. Set the filled bundt pan on the sheet pan. Bake the cake. Bake for 50 to 55 minutes, or until a skewer inserted into the cake comes out with a few crumbs attached. Cool on a wire rack for 10 minutes.
Meanwhile, make the bourbon butter glaze. Place the butter in a small, heavy-bottomed sauce pan over medium-heat. Cook, stirring continuously, until the butter is melted. Whisk in the whiskey until combined. Remove from heat and whisk in the sugar until just combined—at this point, the glaze will seem thick and sugary, but that's totally okay, I promise!
Assemble the Kentucky bourbon butter cake. Use a wooden skewer to poke holes all over the bottom of the bundt cake. Use a pastry brush to brush half of the bourbon butter glaze on the cake, making sure that the glaze seeps through the holes. Place a serving plate or second wire rack over the bundt cake and flip the cake so that the glazed part is now the bottom part of the cake. Turn the cake out of the pan and brush the top of the cake with the remaining glaze. Serve and store. Serve the cake warm, or at room temperature. The assembled Kentucky bourbon butter cake can be stored at room temperature, under a cake dome or a large bowl turned upside down, for up to 3 days. Press a sheet of plastic wrap against any cut surfaces to prevent the cake from drying out.