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 tsp 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.