Preheat oven to 350 F; line two baking sheets with parchment paper or Silpat.

In the bowl of an electric mixer fitted with the paddle attachment, beat together the butter, sugar, salt, and vanilla on medium-high speed until smooth, about 2 minutes. Beat in flour, beginning on low speed and increasing to medium-high, until a dough forms.

Roll dough into 12 equal size balls, placing 6 on each cookie sheet.

Bake for 10 minutes, removing from oven and using a ½ tablespoon measuring spoon make an indentation in the middle of each cookie. Return to oven and bake for another 2-3 minutes. Remove to a wire rack to cool.

When cookies are cool, make the filling. Place the chocolate and butter in a small microwave-safe bowl. Microwave for 20 seconds at a time, stirring in between each time until chocolate is smooth. (When it's almost melted all the way I just do 10 seconds, you don't want to get it too hot.) Allow to cool slightly. Use a small spoon to fill the thumbprints with the chocolate. Let set for about 20 seconds or so then top with sprinkles if using. Let cool completely until set then enjoy! (You can place it in the fridge or freezer until it's set to speed up the time.)