Preheat the oven to 350ºF. Coat a 9x13 baking pan with cooking spray.
In a medium bowl, whisk together flour, cocoa, baking powder, and salt. Set aside.
In a large bowl, whisk together eggs, sugar, butter, and vanilla and beat until smooth. Stir in dry ingredients, mixing well. Fold in chocolate chunks. Transfer batter to the prepared pan and bake until a toothpick inserted in the center comes out with moist crumbs clinging, about 25 minutes.