In a bowl, with an electric mixer on high speed, beat butter, granulated sugar, and brown sugar until smooth.
Beat in egg yolks and vanilla until blended, scraping sides of bowl as needed.
In another bowl, mix flour and salt.
With the mixer on low speed, beat flour mixture into butter mixture until well blended.
Place the turbinado sugar in a shallow bowl.
Shape dough into 1-inch balls and roll in the turbinado sugar to coat.
Place the cookies 1 inch apart on buttered cookie sheet. You can skip coating the sheet with butter and use parchment paper instead.
Press your thumb into the center of each cookie to make a 1/2 inch indentation.
Refrigerate cookies for 30 minutes or longer.
Bake cookies in at 325-degree oven until lightly browned, about 10 to 12 minutes; if baking more than one pan at a time, switch pan positions halfway through baking.
Transfer cookies to a rack to cool completely. If the indentations disappear, make them again while the cookies are warm.
Make Chocolate Ganache at this time.
Carefully fill each indentation with about 1 tsp chocolate ganache.
Let stand until ganache is shiny and firm to touch, about 1 hour. You can chill them for about 30 minutes if desired.