Start by preheating the oven to 350 degrees and line two cupcake pans with paper liners. Set aside.
In a large bowl, or the bowl of a stand mixer, add the boxed cake mix, vegetable oil, eggs (or ingredients as instructed on the back of the box) along with the bourbon and vanilla. Mix until smooth. (The batter may look runny because of the bourbon, this is okay.)
Divide the batter into the prepared cupcake pans, filling each cavity no more than ⅔ of the way full.
Bake according to box directions, approximately 19-21 minutes.
Remove the cupcakes from the oven and transfer to a wire rack to let cool.
Once cooled slightly, poke the tops of the cupcakes with a toothpick and use a pastry brush to brush bourbon over them.
Add the butter to a stand mixer bowl and beat on medium speed for 2-3 minutes, until the butter is light and fluffy.
Add in the powdered sugar ½ to 1 cup at a time until fully combined.
Then add in the bourbon, salt and milk and mix until combined. If your frosting is too thin, add in more powdered sugar ¼ cup at a time until it reaches the desired consistency.
Transfer frosting to a large piping bag fitted with a large star tip. Pipe the frosting onto the cupcakes.
Optionally, fill Shot Tops with bourbon and place into the frosting. Or decorate with sprinkles.