Position the rack in the center of the oven and preheat the oven to 375°F To prepare the muffin pans, spray the holes and the rims around them with the nonstick spray or fill the holes with the paper muffin cups, if using them. If using the silicone muffin pans, spray them as directed and set on a baking pan.
Whisk the cornmeal,flour,sugar,baking soda,baking powder and salt in a medium bowl until well mixed. Set aside.
In a large bowl, whisk the eggs until lightly beaten, then whisk in the buttermilk, sour cream, and melted butter until smooth. Stir in the cornmeal mixture with a wooden spoon until incorporated.
Fill in the prepared tins three quarters full. Use additional greased tins or oven safe ramekins for any leftover batter, or reserve the batter for a second batch. Bake for 22 minutes or until the muffins have a bumpy, rounded top, and a toothpick inserted into one of the muffins comes out clean.
Set the pan on a wire rack to cool for 10 minutes. Gently rock each muffin back and forth to release it. Remove them from the pan and cool for 5 minutes more on the racks. If storing or freezing, cool completely, and then store in airtight containers or freezer proof bags. These muffins will stay fresh up to 24 hours at room temperature, and up to 1 month in the freezer.