Preheat the oven to 350 degrees . Line an 8-inch- square baking pan with foil, leaving a 2-inch overhang. Coat the foil with cooking spray. In a medium saucepan, melt the butter over medium. Off heat, whisk in the sugar, vanilla and salt. Whisk until creamy, about 1 minute. Add the eggs, one at a time, whisking well between additions. Add the cocoa powder and flour; stir until just blended. Pour into the pan.
Bake the brownies until a tester inserted into the center comes out with moist crumbs attached, 40 to 45 minutes. Cool in the pan set on a wire rack. Using the foil overhang, lift the brownies out of the pan. Cut into squares.