The Fudgiest Brownies on Earth

Adjust oven rack to middle position and preheat to 350°F. Line an 8×8 baking dish with foil and lightly coat with cooking spray.

Place butter, bittersweet chocolate, and unsweetened chocolate in double boiler bowl. Gently heat until just melted, stirring occasionally. Remove from heat and let cool slightly.

Using a hand mixer, whisk sugar, eggs, salt, and vanilla in large bowl until light. Whisk in chocolate mixture until thick and glossy. Add flour and cocoa and stir until just combined.

Pour into prepared pan and bake until just set, about 30 minutes. Let cool completely in pan before gently lifting them out using the foil overhang.