Prepare the cookie dough. In a bowl, combine the flour, cocoa powder, salt, and baking soda. Stir to blend and set aside. In a mixing bowl with an electric mixer, beat the melted butter with brown sugar and granulated sugar until smooth. Beat in the egg and egg yolk. Beat in vanilla. Slowly beat the dry ingredients into the butter and sugar mixture until well blended. Fold in chocolate chips.

Chill the dough. Cover the bowl and transfer the dough to the fridge; chill for at least 1 hour, or until firm.

Prepare the oven and pan. Position a rack in the center of the oven and heat it to 350°F. Line a baking sheet with a silicone mat or parchment paper.

Scoop and bake. Using a cookie scoop or tablespoon, arrange mounds of dough onto the baking sheet. Leave about 2 inches between cookies. Bake for 12 to 15 minutes or until set.

Cool the cookies. Let the cookies cool for a few minutes on the baking sheets, then move them to a rack to cool completely.