Preheat oven to 375°F. Line 10 cups in a standard muffin tin with paper liners. Set aside.
In a large bowl, whisk together buttermilk, sugar, melted butter, and egg.
In a medium bowl, sift together flour, cornmeal, baking powder, and salt.
Use a fork or wooden spoon to gradually add the flour mixture to the wet ingredients, stirring just until combined. Do not over-mix.
Divide the batter evenly among the prepared muffin cups, filling each cup to the top. Put water in the empty cups (this will help the muffins bake evenly). Bake for 15-18 minutes, or until a toothpick inserted in the center comes out clean.
Notes
Use afinely-ground yellow cornmeal. Coarse cornmeal will give the muffins a more grainy (rather than soft) texture.
Let the batter rest for 5-10 minutes before baking, if you have the time. This allows the baking powder to activate and incorporate more air.
Don’t over-mix the batter or it will become too dense, and you'll end up with dry muffins.
To check if the muffins are done, insert a toothpick into the center and make sure it comes out clean. Don't cook them for too long, or they can dry out.
Use full-fat buttermilk rather than regular milk. It adds a subtle tangy flavor, it keeps the muffins moist, and it yields a tender crumb. If you don't have buttermilk in your fridge, you can make your own at home by placing 1 tablespoon of white vinegar or lemon juice in a liquid measuring cup and adding enough milk to measure 1 cup. Stir, then let stand for 5 minutes.