Preheat oven to 375℉ and lightly grease muffin cups or line with muffin papers.
Add egg, bananas, melted butter and sugar to the blender pitcher and secure lid.
Blend on “Med” manual setting for 10 seconds. Select “Cancel” to stop blending.
Add salt, baking soda, baking powder and 1 1/3 cups all-purpose flour to the blender pitcher and secure lid.
Blend on “Low” manual setting for 25 seconds. Select “Cancel” to stop blending.
Spoon batter into prepared muffin cups until cups are ⅔ full.
In a small bowl, mix together brown sugar, 2 tbsp all-purpose flour and cinnamon. Cut 1 tbsp butter into mixture until it resembles cornmeal. Sprinkle on top of muffins.
Bake in preheated oven for 16-20 minutes, until golden brown and a toothpick inserted into center of muffin comes out clean.