Preheat oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, whisk together flour, baking powder, baking soda, and salt.
In another bowl, mix mashed bananas, sugar, egg, and melted butter.
Stir the banana mixture into the flour mixture until just combined. Fold in chocolate chips.
Divide the batter evenly among the muffin cups.
In a small bowl, mix together brown sugar, flour, and cinnamon for the streusel topping. Cut in the melted butter until mixture resembles coarse crumbs. Sprinkle over muffins.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.