Preheat oven to 350°F. Grease or line a 9x13-inch baking dish with parchment paper.
In a large mixing bowl, whisk together melted butter, granulated sugar, brown sugar, and vanilla until combined.
Add the eggs one at a time, beating well after each addition.
In a separate bowl, whisk together flour, cocoa powder, baking powder, and salt.
Gradually stir the dry ingredients into the wet ingredients until just combined—do not overmix.
Fold in 1 cup of the chocolate chunks or chips.
Spread the batter evenly into the prepared pan and sprinkle the remaining chocolate chunks on top.
Bake for 30–35 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs.
Let cool completely in the pan before cutting into squares.