Preheat oven to 375 degrees F (190 degrees C). Spray 10 muffin cups with cooking spray.
Whisk 1 1/2 cups sifted whole wheat flour with baking soda, baking powder, salt, and 1/2 teaspoon cinnamon in a large bowl. Mix together mashed bananas, 1/2 cup melted unsalted butter, egg, white sugar, and vanilla extract in a separate bowl. Gently fold banana mixture into dry ingredients just until moistened; do not overmix. Spoon batter into prepared muffin cups, filling them about 3/4 full.
Mix dark brown sugar with 2 tablespoons whole wheat flour and 1/2 teaspoon cinnamon in a bowl, stirring until combined; mash 1 tablespoon unsalted butter into the mixture with a fork until streusel topping resembles crumbs. Sprinkle topping over muffins.
Bake muffins in the preheated oven until a toothpick comes out clean or with moist crumbs, about 20 minutes.
