Adjust oven rack to top center position. Place stick of butter into 10-inch oven-proof skillet and place in the oven. Preheat oven with skillet in it to 425°F.
Meanwhile, whisk the cornmeal, flour, sugar, baking powder, baking soda and salt in a large mixing bowl.
Measure the buttermilk and lightly beat the egg into the buttermilk. Pour into the dry ingredients and stir to combine.
When the oven is preheated, carefully remove the skillet and pour the butter into the mixing bowl. Stir with a wooden spoon until butter is incorporated into batter. Do not overmix. Pour batter evenly into the hot skillet. Bake 12-15 minutes, until edges are golden and center is set.
**If using a baking pan, do not preheat baking pan. Instead, melt butter in a microwave safe bowl then use some to brush/grease the pan and pour the rest into the batter. Same if making individual muffins. Baking times will vary.