Preheat the oven. Set to 350°F and line a muffin tin with paper liners.
Prepare the batter. In a large bowl, cream together the melted butter, sour cream, and mashed bananas until smooth. Whisk in the eggs and vanilla. In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gently fold the dry ingredients into the banana mixture until just combined—no overmixing!
Make the topping. In a small bowl, combine the flour, pecans, brown sugar, and salt. Drizzle in the melted butter and stir until the mixture becomes pebbly and crumbly.
Assemble. Spoon the batter evenly into the muffin liners, filling about ¾ full. Sprinkle each generously with the pecan crunch topping.
Bake. Bake for 20–22 minutes, or until a toothpick inserted in the center comes out clean.
Cool & enjoy. Transfer muffins to a wire rack to cool slightly (though warm from the oven is irresistible).