Preheat oven to 325°F. Line an 8x8x2″ glass baking dish with foil, pressing firmly into pan and leaving a 2″ overhang. Coat foil with nonstick spray; set baking dish aside. Melt butter in a small saucepan over medium heat. Let cool slightly. Whisk sugar, cocoa, and salt in a medium bowl to combine. Pour butter in a steady stream into dry ingredients, whisking constantly to blend. Whisk in vanilla. Add eggs one at a time, beating vigorously to blend after each addition. Add flour and stir until just combined (do not overmix). Scrape batter into prepared pan; smooth top. Bake until top begins to crack and a toothpick inserted into the center comes out with a few moist crumbs attached, 25-30 minutes. Transfer pan to a wire rack; let cool completely in pan. Using foil overhang, lift brownie out of pan; transfer to a cutting board. Cut into 16 squares.