Preheat oven to 350 degrees. Line large sheet pan with parchment paper.
In a large mixing bowl using an electric hand mixer, cream together butter, brown sugar, and granulated sugar until creamy. Mix in egg & vanilla until just combined.
Sift in flour, cocoa powder, baking soda, and salt. Add in chocolate chips. Fold all together until well mixed and a firm dough forms.
Using a large cookie scoop form dough balls and place on prepared sheet pan. Place a small piece of parchment on dough ball and smash down with the bottom of a glass to 1 inch thickness.NOTE: Use a small cookie scoop to form mini versions of this cookie.NOTE: For additional chocolate flavor, smash additional chocolate chips on top of the flatten cookie dough.
Bake in preheated oven for 10 minutes until cooked through. Edges should be a little crispy and top of cookie should be firm. Cool on sheet pan for 5 minutes before removing to a wire rack to finish cooling and setting up. Enjoy.