• Preheat oven to 350 degrees. Line a baking sheet with Silpat or parchment paper.
• Using a stand mixer beat the butter and sugar together until soft and creamy. Add 1 egg yolk, cocoa powder, and vanilla and continue to mix.
• In a separate bowl whisk flour, baking soda, and salt.  
• Add flour mixture to butter mixture and mix until just combined and a dough forms.
• Scoop dough with a mini ice cream scoop about 2 teaspoon and roll into a ball.  Place on cookie sheet and press the center down with your thumb.
• Cook for 7 minutes.  They may not seem done but I like mine undercooked a bit.  Gently press the center down again with your thumb to make sure you have a good indentation.  Let cool before removing from sheet.

Buttercream Filling

• Mix all buttercream ingredients together in a stand mixer.  Put icing into a piping bag or use a big ziploc with the corner tip cut off.  Pipe a small amount of icing in center of cooled cookies.  Top with sprinkles.