Preheat oven to 350°. Whisk together the first 6 ingredients. In a separate bowl, whisk bananas, oil and egg until blended. Add to flour mixture; stir just until moistened. Fold in chocolate chips.
Fill greased or paper-lined muffin cups three-fourths full. Bake until a toothpick inserted in center comes out clean, 20-25 minutes. Cool 5 minutes before removing from pan to a wire rack. Serve warm.
Optional streusel topping: Combine 1/2 cup sugar, 1/3 cup all-purpose flour and 1/2 teaspoon ground cinnamon; cut in 1/4 cup cold butter until crumbly. Before baking, sprinkle over filled muffin cups; bake as directed.