In large bowl, mix mashed bananas, melted and cooled butter, 1 cup sugar, the egg, milk and vanilla to combine. In another small bowl, stir together 1 1/2 cups flour, the baking powder, salt and 1/2 teaspoon cinnamon. Fold banana mixture into flour mixture just until combined, taking care not to over mix. Spread batter in loaf pan, smoothing top. Sprinkle streusel evenly over top.