In the bowl of a stand mixer, cream together the butter, brown sugar, and granulated sugar for 3–4 minutes until light and fluffy.
Add the eggs one at a time, followed by the vanilla extract.
In a separate bowl, whisk together: flour, cocoa powder, cornstarch, baking soda, baking powder, and salt.
Gradually add the dry ingredients to the butter mixture, mixing just until combined.
Fold in all three chocolates.
Cover the dough and refrigerate for at least 1 hour. Overnight is best though.
This helps create thick, chewy cookies while keeping the edges from spreading too quickly.
Preheat the oven to 350°F.
Line two baking sheets with parchment paper.
Using a large cookie scoop (about 1/3 cup of dough), portion the dough into 10 large cookies.
Place only 4–5 cookies per baking sheet, leaving plenty of room for spreading.
Press a few extra chocolate chunks into the tops before baking for a bakery-style finish.
Bake for 12–14 minutes.
The edges should be set while the centers still look slightly underbaked.
As soon as the cookies come out of the oven, use a large round cookie cutter or bowl to gently swirl around each cookie to create perfectly round edges.
Allow the cookies to cool on the baking sheet for 10 minutes before transferring to a cooling rack.