Using an electric mixer, beat butter and sugars until light and fluffy, about 2-3 minutes, scraping sides of bowl as needed.
Add egg yolk and vanilla bean paste or extract and beat just until well combined. Add the flour and salt and beat on low speed until just incorporated.
Line two large baking sheets with parchment.
Scoop cookie dough into approximately 1" balls and roll in sugar or nonpareils until completely covered. Place on prepared baking sheet a couple inches apart – you should get about 20 on a half size sheet pan. Use your thumb or the handle end of a wooden spoon to create an indent in the center of each dough ball.
Transfer the first completed cookie sheet to freezer to chill while you complete the second sheet.
Preheat oven to 375 degrees.
Once oven is preheated, remove first baking sheet from the freezer and transfer to the oven. Bake for 12-14 minutes, until cookies are just set and lightly brown around the edges. Place second cookie sheet of dough balls in freezer to chill while first sheet is baking.
Remove from oven and immediately use a rounded teaspoon or end of wooden spoon to re-indent the thumbprints, if needed. Allow to cool on baking sheet for 5 minutes before removing to wire rack to cool completely before filling.
Combine chocolate and butter in a microwave safe bowl. Microwave on 50% power for one minute, stir and return to microwave for 30 second intervals at 50% power, stirring after each interval, until chocolate is completely melted and smooth.
Fill pastry bag or zip lock bag with ganache. Cut a small hole in the tip of the bag and pipe ganache to fill the center of each cookie. Allow to set up at room temperature for about 1 hour.
Notes
Store at room temperature in an air tight container for up to one week.