Fudgy One-Pot Brownies

unsalted butter, plus butter for greasing the baking pan (optional)

Nonstick cooking spray (optional)

unsweetened chocolate

⁄2 cups unsweetened Dutch-processed cocoa powder

1⁄2 cups granulated sugar

⁄2 teaspoons kosher or coarse salt

pure vanilla extract

large eggs

1⁄2 cups all-purpose flour