Preheat the oven to 425 degrees F. Spray a 9-inch-square baking pan with nonstick cooking spray.
In a large bowl, whisk together the cornmeal mix, buttermilk and egg, whisking until no lumps remain. Pour into the prepared pan. Bake until a wooden pick inserted in the center comes out clean, 15 to 20 minutes. Cool completely in the pan.