Preheat the oven to 400 degrees F; coat a 9-by-13-inch baking dish with cooking spray. Whisk the cornmeal, flour, sugar, baking soda and salt in a large bowl. Whisk the buttermilk, melted butter and eggs in a medium bowl; add to the cornmeal mixture and whisk until combined.
Spread the batter evenly in the baking dish. Bake until golden brown and the top springs back when gently pressed, 12 to 15 minutes. Remove from the oven and let cool completely.
Invert the cornbread onto a cutting board and cut into 1/2-inch cubes. Spread out on a baking sheet and leave at room temperature overnight to dry out.