Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on the sides for easy removal.
In a large mixing bowl, whisk together the melted butter and sugar until smooth and glossy, about 1 minute.
Add the eggs one at a time, beating well after each addition until the mixture becomes thick and pale, about 2-3 minutes total.
Stir in the vanilla extract, then sift in the cocoa powder, flour, salt, and baking powder. Fold gently with a spatula until just combined with no dry streaks visible.
Reserve 1/2 cup of chocolate chips for topping, then fold the remaining 1 cup into the batter, being careful not to overmix.
Pour the batter into the prepared pan, spreading it evenly to the corners. Sprinkle the reserved chocolate chips over the top.
Bake for 28-32 minutes, until the edges are set and a toothpick inserted 2 inches from the edge comes out with a few moist crumbs. The center should still look slightly underdone.
Cool completely in the pan on a wire rack for at least 2 hours before lifting out using the parchment overhang and cutting into 16 squares.
