Preheat the oven to 325 F. Grease and flour a 10-cup bundt pan, set aside.
In a large bowl whisk together the flour, baking powder, baking soda, and salt.
in a separate large bowl add 2 cups of sugar and 1 cup of butter. Beat with an electric mixer or stand mixer until the mixture is light and fluffy, 3 to 5 minutes.
Add in the eggs, buttermilk, 2 teaspoons of vanilla extract, and 4 tablespoons of bourbon. Mix until well combined.
Gradually add in the flour mixture and mix until just evenly combined.
Pour the batter into the prepared cake pan and spread evenly. Bake in the preheated oven for 50 to 60 minutes, until a toothpick inserted in the center comes out clean with no wet batter. Leave the cake in the pan while you make the glaze.
Once the cake is baked, add the remaining ½ cup of butter to a medium saucepan over medium. Once melted, add the remaining 1 cup of sugar and 2 tablespoons of water. Cook, whisking often, until the mixture comes to a boil. Continue cooking until the sugar is dissolved, about 1 minute.
Remove from the heat and stir in the remaining 2 teaspoons vanilla extract and 2 tablespoons bourbon until combined.
With the cake still in the pan, use a skewer or the handle of a wooden spoon to poke holes all over the bottom of the cake. Pour about ½ of the glaze over the cake, allowing the glaze to soak into the cake. If the liquid pools in places poke more holes to let the glaze soak in. Let the cake sit for about 30 minutes to fully soak in.
Turn the cake over onto a cooling rack on top of a baking pan. Drizzle the rest of the glaze over the top of the cake (the baking pan will catch any glaze that drips down from the cake). Let the glaze set before slicing and serving the cake.