Preheat your oven to 350˚F. In a large microwave-safe bowl, melt the butter and 1 cup of chocolate chips (about 1 minute). Let sit 2–3 minutes, then whisk until smooth.
Whisk in granulated sugar, brown sugar, vanilla extract, and salt.
Whisk in eggs until well combined and glossy.
Sift in flour and dark cocoa powder. Fold with a spatula just until combined. Stir in the remaining chocolate chips.
Line a 9-inch square pan with parchment paper and lightly spray with nonstick spray. Pour in the batter and smooth the top. Bake 35–40 minutes, until a toothpick comes out with just a few moist crumbs.
Cool completely before cutting (they slice best chilled). Bring to room temperature before serving for the perfect texture.
