Preheat oven to 350°F. Line a 9x13-inch pan with foil and spray with nonstick cooking spray.
Place chocolate and butter in a large microwave-safe bowl. Heat on high power in 30 second increments, stirring between each, until chocolate and butter are melted and smooth (about 2 minutes, depending on your microwave).
Stir sugar and cocoa into chocolate and butter mixture using a spatula or wooden spoon. Stir in eggs and vanilla, then stir in salt. Mixture will be grainy looking but smooth. Stir in flour carefully.
Pour batter into prepared pan. Bake for 25-35 minutes, or until the top is no longer glossy and a toothpick comes out with just a few crumbs when placed 2-inches from the side of the pan. Cool completely before slicing. I prefer mine baked only 25 minutes; if you want yours less fudgy, bake longer.
Video
Notes
Tip: add 1 1/2 cups of chocolate chips or chopped nuts to the batter after stirring in the flour or frost with chocolate frosting.