Heat oven to 350°F; line sheets with parchment or silicone mats.
Whisk flour, cocoa, baking soda, and salt in a medium bowl; set aside.
Beat butter and both sugars with an electric mixer until fluffy, about 2-3 minutes.
Beat in egg, yolk, and vanilla until smooth.
Gradually mix in dry ingredients on low until just combined.
Fold in chocolate chips evenly.
Roll heaping tablespoons into balls; place 2 inches apart on sheets.
Bake 9-11 minutes until edges set and centers look slightly underdone.
Cool on sheets 10 minutes, then on a rack; cookies firm as they cool.