Preheat the oven to 400°F. Line two baking sheets with parchment paper or a silicone baking mat.
In the bowl of a stand mixer fitted with the paddle attachment, combine 1 cup butter (softened), 3/4 cup brown sugar, and 1/2 cup granulated sugar. Mix on low speed to incorporate, then increase to medium and beat until smooth, about 1 minute.
Add 2 large eggs and 1 teaspoon vanilla extract. Beat until just combined, about 30 seconds.
Add 2 1/2 cups all-purpose flour, 1/2 cup cocoa powder, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1/2 teaspoon salt. Mix on low speed until just combined, scraping down the sides of the bowl as needed.
Fold in 2 cups milk chocolate chips and 1 1/2 cups dark chocolate chunks using a rubber spatula or wooden spoon.
Divide the dough into 8 to 10 equal portions. Roll each portion into a ball using your hands and place them on the prepared baking sheets, spacing them 4 to 5 per pan.
Bake for 11 to 14 minutes, or until the tops are set.
Let the cookies cool on the baking sheet for 15 minutes before serving warm or transferring to a wire rack to cool completely.
Notes
For smaller cookies portion into 24 equal-sized dough balls, about 2 ounces each. Bake at 400 degrees for about 8 minutes.
For extra tall cookies, chill or freeze the dough balls before baking to reduce spreading and achieve chunky, bakery-style cookies.
Freezing Dough Balls: Freeze pre-portioned dough balls on a baking sheet, then transfer to a freezer-safe bag. Bake directly from frozen at 400°F, adding 1–2 minutes to the baking time.