Preheat the oven to 425 degrees F, and lightly grease or line a muffin pan with papers.
Place the flour, sugar, cornmeal, baking powder, and salt in a large bowl and whisk to combine.
Melt the butter in a large liquid measuring cup, add the milk, oil, and eggs, and whisk to combine.
Pour the liquid ingredients into the dry, and stir together until just barely combined.
Divide the batter equally between all 12 wells of the pan.
Bake the muffins for 5 minutes at 425 degrees F, then turn the oven temperature down to 350 degrees F and continue to bake until a toothpick inserted in the thickest part of a muffin comes out clean or with a few moist crumbs (approx. 10 to 15 minutes).