Preheat the oven to 350° F. Line a cookie sheet with parchment paper or a silicone baking mat and set aside.
Cream together the butter, brown sugar and granulated sugar until smooth and creamy.
Add the egg and egg yolk, corn syrup and vanilla extract to the mixer, mix until well incorporated.
Add the flour, cocoa powder, cornstarch, baking soda and salt to the mixer. Mix until a soft dough forms.
Fold in the peanut butter chips until evenly distributed.
Scoop 3 tbsp of dough and loosely gather the dough together, leaving some jagged edges on top for texture. The cookies will spread some but not a whole lot while baking so it is best if they are not packed together in a ball.
Arrange on the prepared cookie sheet and bake at 350° F for 9-11 min.
Allow the cookies to cool on the pan for 10 min before transferring to a wire cooling rack.
Store in an airtight container for up to 3 days.