How to Make It
Preheat the oven to 350°F. Line an 8x8-inch baking pan with aluminum foil, allowing a 1-inch overhang; lightly spray the foil with nonstick spray. Set aside.
Combine the butter and chocolate chips in a medium saucepan over low heat. Heat, stirring frequently, until butter and chocolate are melted and evenly combined. Turn off the heat; whisk in the sugars and cocoa powder until combined. Whisk in the eggs, one at a time, until incorporated. Whisk in the vanilla. And finally, whisk in the cornstarch and salt. Stir in any additional mix-ins you like.
Scrape the batter into the prepared pan and bake until top begins to crack and a toothpick inserted into the center of the brownies comes out with just a few moist crumbs, about 30-35 minutes. Cool brownies completely in pan on a wire wrack. Use the aluminum foil overhang to lift the brownies from pan and transfer to a cutting board; cut into 16 squares.
We tested this recipe using Ghirardelli Semi-Sweet Grand Chips with great success.