Melt butter in a medium-sized pot. Remove from heat. Stir in brown sugar and granulated sugar until well combined.
Add in egg, egg yolk and vanilla. Mix until lighter in color.
Add in the flour, cocoa powder, baking soda, and salt. Stir well to ensure a smooth, well-stirred batter.
Preheat oven to 325.
Let batter it sit 10 minutes or so to let the flour soak into the rest of the batter. (The batter should be warm and obviously sticky. As it sits and cools, the batter will thicken to a more obvious dough. This is when you would scoop the dough into cookies.)
Line two baking sheets with parchment paper and scoop cookie dough onto prepared pans, using a 1 1/2 tablespoon cookie scoop.
Bake 8-9 minutes, rotating sheets halfway through baking. When you pull your cookies out of the oven, they will look cooked around the edges and undercooked in the center.
Leave the cookies on the hot baking pans for 5-7 minutes or until you can remove them without falling apart. Place onto cooling racks and cool to room temperature before storing in airtight containers.
Best eaten in the first 24 hours after baking.