Pre-heat oven to 350°F (180°C), grease and flour a large or medium sized muffin tin or 9 inch (25 cm) loaf pan.
In a large bowl mix together with a whisk or spatula, the mashed bananas, melted cooled butter, egg, sugar and vanilla, until well combined.
In a medium bowl whisk together flour, baking soda and salt.
Add dry ingredients to wet mixture and mix just until combined (I usually stir approximately 15-17 times)
Spoon into prepared muffins cups (a little more than 3/4 full) or loaf pan, top with the streusel topping. Bake for approximately 20-25 minutes (depending on the size of the muffin tin) or 30-35 for the loaf, check for doneness with a tooth pick. Let cool in pan. Enjoy.
PECAN STREUSEL TOPPING
In a small bowl mix together with a fork brown sugar, flour and pecans then add the butter and combine with a pastry blender or fork until it resembles coarse crumbs.
For the muffins I got 5 large muffins so I assume you could get 10 medium.