Step 1

Preheat oven to 400°. Line a 13x9" baking pan, preferably metal, with foil, pressing firmly into corners and smoothing creases. Lightly coat with nonstick spray; set aside. Whisk flour, cocoa, and salt in a medium bowl (if cocoa powder is very lumpy, sift mixture).

Step 2

Heat chocolate and butter in a heatproof bowl set over a saucepan of simmering water (bowl should not touch water), stirring constantly, until melted and mixture is smooth. Remove from heat. Whisk eggs, both sugars, and vanilla in another medium bowl until smooth, then whisk into chocolate mixture. For a shiny, crackly top, whisk mixture an additional 10 seconds. Add dry ingredients and whisk just until flour disappears and mixture is smooth and glossy. Scrape into prepared pan and smooth top.

Step 3

Place pan in the oven and reduce oven temperature to 350°. Bake brownies until lightly puffed in the center and a tester inserted into the center comes out greasy but mostly clean with only a few crumbs attached, 25–30 minutes. Let cool.

Step 4

Invert pan onto a wire rack, remove foil, then invert onto a cutting board right side up. Cut into squares and serve topped with scoops of ice cream and desired toppings.

Step 5

Do Ahead: Brownies can be baked 2 days ahead. Store tightly covered at room temperature.