Using paddle attachment, add butter to bowl of standing mixer.
Mix for a few seconds to break up butter, do not over mix.
Add white and brown sugar, mix on medium speed until incorporated.
Add chocolate chips and mix.
In a separate bowl mix together: flour, cocoa, salt and baking soda.
Add dry ingredients to mixer bowl an mix until sandy looking.
In a small bowl, mix together: eggs and egg yolk.
Add the beaten eggs to the rest of the dough and mix until dough pulls away from the sides of the bowl.
Measure dough into 125 gram balls, it should make 13 balls.
Freeze at least 90 minutes.
Preheat oven and cookie sheet to 356 degrees F. convection.
Bake frozen dough balls on hot cookie sheet for 14-18 minutes.
Remove sheet from oven and let cool for 15 minutes before eating.
