Preheat oven to 350F and line cookie sheets with parchment paper.
In large, micro-wave safe bowl, combine butter and chocolate chips. Microwave for 30 seconds and stir, repeating at 15 second increments until chocolate and butter are completely melted.
Remove from microwave and allow to cool 10 minutes before proceeding.
Once butter/chocolate mixture has cooled, stir in sugars, eggs and vanilla.
In separate, medium-sized bowl, whisk together flour, cocoa powder, cornstarch, baking powder, baking soda, and salt.
Gradually stir flour mixture into butter mixture until completely combined.
Drop cookie dough by rounded 1½" scoop onto parchment paper, pressing down slightly. Bake 9-10 minutes, allow to cool on cookie sheet for 5 minutes and then transfer to cookie rack to cool completely.