Spread butter along bottom and sides of a 3-quart baking dish. Add sugar, cornmeal, and 2 1/2 cups cold water; stir well with a whisk. Bake at 350° for 30 minutes. Remove from oven, and stir mixture carefully with a fork or whisk until smooth; bake an additional 15 minutes.
Remove from oven, and stir until smooth. Add chocolate; stir until chocolate melts completely. Garnish with zest, and serve warm or at room temperature with a slice of orange, if desired.