Lightly spoon the cornmeal and flour into measuring cups; level with a knife. Combine cornmeal, flour, baking powder, baking soda, and salt in a large bowl, stirring with a whisk. Combine buttermilk, butter, and egg. Pour buttermilk mixture into cornmeal mixture; stir just until moist.
Spoon the batter evenly into 6 muffin cups coated with cooking spray. Bake at 400° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool on a wire rack.
I added 2 TB of sugar and used egg substitute instead of an egg. Have made these twice and liked them both times. Makes a big fluffy muffin with a good crumble and a decided corn taste. Perhaps the sugar makes the difference between my results and other reader's.