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 or whisk 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 finely-ground yellow cornmeal for soft texture, not coarse cornmeal.
Full-fat buttermilk creates the most tender, moist muffins. To substitute: add 1 tablespoon of vinegar or lemon juice to a measuring cup, then add milk to equal 1 cup. Let sit 5 minutes.
Don't overmix the batter or muffins will be dense and dry. Stir just until combined.
Let batter rest 5-10 minutes before baking for fluffier muffins (optional but helpful).
Fill muffin cups all the way to the top for tall, bakery-style domed tops.
Add water to empty muffin cups for even baking.
Bake just until a toothpick comes out clean. Don't overbake or muffins will dry out.
Remove from pan immediately and cool on a wire rack.
For less sweet muffins, reduce sugar to 1/4 cup or 2-3 tablespoons.
Store in an airtight container at room temperature 1-2 days, refrigerator up to 1 week, or freeze up to 3 months.