Before you begin mixing, prepare your pans and oven. Preheat the oven to 350°F. Grease your cake pans thoroughly and line the bottoms with parchment paper to prevent sticking.
In a medium bowl, stir together the flour, baking powder, and salt. Mix until everything is evenly distributed then set the bowl aside.
In a large bowl, beat the butter, oil, and sugar together. Continue mixing for about 3–4 minutes, until the texture becomes light, creamy, and slightly fluffy.
Add the eggs one at a time, mixing well after each addition so the batter stays smooth. Once incorporated, blend in the vanilla extract and bourbon.
Gradually combine the dry ingredients with the wet mixture. Add the flour mixture in three portions, alternating with the milk. Begin and finish with the flour mixture. Mix gently and only until combined to keep the cake tender.
Evenly divide the batter between the prepared pans. Place them in the oven and bake for 30–35 minutes. Check for doneness by inserting a toothpick into the center if it comes out clean, the cakes are ready. Let them cool in the pans for 10 minutes before transferring to a wire rack to cool completely.
To make the buttercream, beat the butter until smooth and creamy. Slowly add the confectioners’ sugar, mixing continuously. Add the vanilla extract, bourbon, vanilla bean seeds, and salt, and then beat until the frosting becomes light and fluffy.
Once the cakes are completely cooled, place one layer on a serving plate and spread buttercream over the top. Add the second layer and frost the top and sides evenly. Slice and enjoy.