Preheat oven to 400 degrees F and line 12 (1/3-cup) muffin cups with paper liners.
Into a bowl, sift together flour, sugar, baking powder, baking soda, and salt. Melt butter and in a small bowl whisk together with sour cream, egg, and vanilla. Stir butter mixture (and additional ingredients; *see note, below) into flour mixture until just combined. Divide the batter among muffin cups (this batter is thick and will not run easily like other muffin batters) and bake in middle of oven until golden and a tester comes out clean, about 20 minutes.
Becoming a muffin master is easy. *Flavor the basic muffin batter below by adding, for example: 1 chopped banana and 3/4 cup semisweet chocolate chips; or 1 cup toasted almonds, 3/4 cup dried cranberries, and 1/4 teaspoon almond extract; or 1 cup chopped drained canned pineapple and 1 cup sweetened flaked coconut; or 1 cup chopped dried apricots and 1 tablespoon poppy seeds.