In a small bowl (or electric mixer if you prefer, but not necessary), use a firm spatula to cream together the butter, salt, and sugars until smooth and incorporated.

Add the egg yolk and vanilla extract. Mix until smooth and creamy.

Stir in flour, baking soda, and cocoa until a dough forms. Add chocolate chips.

Dough will be wet/sticky. Separate into two equal mounds of dough. Press a few extra chocolate chips into the top if desired. Chill dough balls in the refrigerator for 15 minutes. Preheat oven to 350° F while you wait.

Transfer cookie dough balls to a baking sheet lined with parchment or a silicone baking mat. Bake for 8-10 minutes. Let cool on the baking sheet for 10 minutes before transferring to a cookie rack to cool completely.