Cream the butter. Add the discard, brown sugar, and white sugar and continue to mix.
Add the egg, egg yolk, and vanilla and mix until combined.
In a separate bowl mix together the flour, cocoa powder, corn starch, baking soda, and salt. Add to the butter mixture.
Stir in chocolate chunks until just combined.
Portion the dough into balls (about three tablespoons per cookie) and place on a cookie sheet lined with parchment paper or a silicone mat. Place in fridge for 30 mintues.
While in the fridge, preheat oven to 375℉. After 30 minutes, remove cookies from fridge and transfer 12 at a time to a new cookie sheet. Bake for 10-12 minutes, then take out of oven and allow to rest on the cookie sheet before removing them to a cookie sheet.