In a medium bowl, sift together the flour, cocoa, cornstarch, baking powder, baking soda and salt. Whisk until combined.
In a separate large bowl, beat together the butter, brown sugar and granulated sugar until there are no more lumps of butter.
Beat the eggs and vanilla extract into the butter mixture.
Mix the dry ingredients into the butter mixture. I typically do this about 1/2 at a time, starting with the mixer on a low speed.
Stir in the chocolate chips/chunks. The dough will be thick and sticky - this is expected.
Cover the bowl with plastic and refrigerate for at least 3 hours or up to 48 hours. This step is mandatory for thick cookies - otherwise they can spread too thin and burn.
When ready to bake, preheat the oven to 375F (190C) and line cookie sheets with parchment or baking mats. Take the dough out of the fridge and let it sit on the counter for 5-10 minutes (this may not be necessary if the dough was only chilling for 3 hours).
For the dough into large balls with about 1/4 cup (4 tablespoons) of dough each. I used a scale and made all my dough balls about 4 ounces or 112 grams in size). Place at least 2.5 inches (5-7 cm) apart on the lined cookie sheets.
Bake 1 sheet at a time in the middle of the preheated oven for about 12-14 minutes, or until the tops look almost set. Keep the other cookie trays of unbaked cookie dough balls in the fridge while the first trays are baking.
Cool the cookie dough balls on the cookie sheets for at least 20 minutes as they will be very warm and gooey. For perfectly round cookies - take a large mug or round cookie cutter that is larger than the cookies. Place it over one cookie at a time a gently swirl a circle around each cookie so that the baked cookie is gently moulded into a round shape.