Preheat the oven to 400 degrees Fahrenheit. Add 12 muffin cups to a tin.
In a large bowl, whisk together the cornmeal, flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk together the egg, Greek yogurt and milk. Melt the butter, then pour both the milk and butter (or oil) into the dry ingredients. Stir until just combined. Pour into the muffin cups (the cups will be very full).
Bake about 15 to 17 minutes until the center has firmed up and is lightly golden, and a toothpick comes out clean. Allow to cool in the muffin tin for 5 minutes, then add to a baking rack. Store any leftovers at room temperature covered with a towel for about 3 days, or in a sealed container at room temperature for about 5 days.