Using an electric mixer, beat butter until well creamed.
Add sugars and beat until light and fluffy, about 30 seconds.
Add egg yolk, vanilla extract, and vanilla paste (if using), and stir well, pausing to scrape down the sides and mixing until ingredients are well-combined.
Add flour, gradually (about ½ cup at a time), pausing to scrape down sides of bowl. Halfway through the addition of the flour, sprinkle in the salt with the beater still running.
Roll cookie dough into approximately 1" balls and roll through the coarse sugar until completely covered in sugar. Place on a wax paper lined plate and use your thumb to create an indent/crater in the center.
Transfer cookie dough balls to freezer and chill for at least 30 minutes (this will keep cookies from spreading too much).
Preheat oven to 375F (190C) and prepare a cookie sheet by lining with parchment paper (if you don't have parchment paper, just use an ungreased cookie sheet).
Once oven is preheated, remove cookie dough from freezer and transfer to prepared baking sheet, spacing cookies at least 1 ½" apart. Bake for 12 minutes.
Remove from oven and immediately use a rounded teaspoon to re-indent the thumbprints, if needed.
Allow cookies to cool while you prepare the ganache filling.
Combine chocolate chips, heavy cream, and butter in a small saucepan over medium-heat. Stir frequently until chocolate is completely melted and mixture is smooth.
Remove from heat and stir in vanilla extract, bourbon, and salt. Stir well.
Allow mixture to sit and cool slightly, about 10 minutes.
Use a spoon to pour chocolate ganache into the crater of each prepared thumbprint cookie. Ganache will set after about 1 hour at room temperature or you can put them in the refrigerator for about 30 minutes to speed up the process (cookies taste best at room temperature though).