Heat oven to 375 degrees. Coat a standard 12-cup muffin tin with nonstick spray or line with paper liners.
Spread pecans on a small rimmed baking sheet and bake until toasted, 6-8 minutes. Remove from oven, add butter and toss until evenly coated. Cool on the sheet while preparing the batter.
Whisk flour, baking powder, baking soda and salt in a large bowl. Whisk banana, sugar, buttermilk, oil, egg and vanilla in a medium bowl until smooth.
Make a well in the center of dry ingredients and pour in wet ingredients. Fold gently with a silicone spatula until no traces of flour remain. Divide batter evenly among muffin cups and sprinkle with buttered pecans.
Bakerotating tin halfway through, until toothpick inserted in the center of a muffin comes out clean, 16-18 minutes. Cool in the tin on a wire rack for 10 minutes, then unmold. Serve warm or at room temperature.
•Spiced muffins: Add 1/2 teaspoon ground cinnamon and 1/4 teaspoon ground cardamom to the flour mixture.
•Nuttier muffins: Substitute toasted walnut or other nut oil for the vegetable oil. Stir 1 cup chopped toasted pecans into the batter before sprinkling the buttered nuts on top.
•Chocolate muffins: Stir 1 cup chocolate chips or finely chopped chocolate into the batter.