Preheat oven to 350°F Grease or line 12 muffin tins with paper liners.
Prepare the crumb topping: In a small bowl, whisk together flour, sugar and salt. Cut shortening into flour mixture until it resembles coarse crumbs. Larger crumbs are a bit more desirable. Stir in the mashed banana and set bowl aside.
Prepare the batter: In a medium bowl, sift together flour, baking soda, baking powder and salt. In a large bowl, whisk together sugar, vegetable oil, buttermilk, egg, bananas and vanilla. Pour flour mixture into banana mixture and stir until batter is just combined. Spoon into muffin tins, filling each 2/3 full. Top each muffin with crumb mixture, dividing it evenly.
Bake for 18-21 minutes, until a tester comes out clean and the muffins spring back when gently touched.