Beat butter and sugar at medium speed with an electric mixer until creamy; add egg, beating just until mixture is blended.
Combine flour, baking powder, and salt; add to butter mixture alternately with milk, ending with flour mixture. Spoon into greased muffin pans, filling two-thirds full.
Bake at 375° for 25 minutes. Remove from pans immediately; cool on wire racks.
Pepper-Cheese Muffins: Reduce sugar to 2 tablespoons; stir in 1 cup (4 ounces) shredded Monterey Jack cheese with jalapeño peppers with flour mixture.
Bacon-Cheese Muffins: Reduce sugar to 2 tablespoons; stir in 1/2 cup (2 ounces) shredded Cheddar cheese and 3 bacon slices, cooked and crumbled, with flour mixture.
Herb Muffins: Reduce sugar to 2 tablespoons; stir in 1 teaspoon instant minced onion, 2 teaspoon dried parsley flakes, 1 teaspoon dried oregano, and 1/2 teaspoon pepper with flour mixture.