In a large microwave-safe bowl, melt the butter completely.
Add the Dutch-process cocoa powder and whisk until smooth and glossy. The mixture should look like a thick chocolate sauce.
Let the mixture cool for 5 to 10 minutes before continuing.
Add the brown sugar and granulated sugar to the bloomed cocoa mixture. Whisk until well combined.
Add the eggs and vanilla and mix until smooth.
In a separate bowl, whisk together the flour, baking soda, cornstarch, and salt.
Gradually stir the dry ingredients into the wet ingredients until just combined.
Fold in the chocolate chips.
Cover the dough and refrigerate for at least 30 minutes, or up to 2 days. Chilling helps the cookies hold their shape and develops deeper chocolate flavor.
When ready, preheat oven to 350°F. Line baking sheets with parchment paper.
Scoop the dough into portions (about 2 to 3 tablespoons per cookie) and place them 2 to 3 inches apart on the prepared baking sheets.
Bake for 10 to 12 minutes, or until the edges are set and the centers still look slightly underdone.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack.