Preheat the oven to 350°F. Line a 9-by-13-inch baking pan with foil, leaving a 2-inch overhang on two sides. Coat the foil with cooking spray. Combine the butter and chocolate in a large saucepan and cook over low heat, stirring with a rubber spatula, until melted. Remove from the heat and let cool slightly.

Add the sugar and eggs to the chocolate mixture and mix with a whisk until smooth. Add the flour, cocoa powder and salt and whisk until combined.

Spread the batter in the pan with the rubber spatula. Bake until a toothpick inserted into the center of the brownies comes out clean, 30 to 35 minutes. Remove the pan from the oven with oven mitts, put the pan on a rack and let the brownies cool completely.

Lift the brownies out of the pan using the overhanging foil, then peel off the foil. Cut the brownies into squares.