Preheat the oven to 350 degrees . Grease a 9-by-9-inch baking pan. In a large bowl, combine 2 cups sugar, 1 1/2 sticks melted unsalted butter and 1/2 teaspoon salt. Whisk in 3 eggs, 1 at a time. Stir in 1 cup unsweetened cocoa powder and 1 cup flour. Transfer the batter to the baking pan and bake until just set, 25 to 30 minutes; let cool.