Stir together all ingredients in a bowl just until moistened; let stand 4 minutes. Using a spatula, spread corn muffin batter into an 8" x 9" rectangle in a lightly greased, parchment paper-lined jelly-roll pan.
Bake at 400° for 10 minutes or until golden; turn oven off, and remove cornbread. Cool. Cut cornbread into 1" cubes. Return cornbread to warm (turned-off) oven. Let stand in oven, with door closed, 1 hour or until croutons are dry and crunchy.