Pre-heat the oven to 350ºF. Spray a 12-cup muffin pan with cooking spray or line with muffin cups.
To make the streusel, combine flour, butter, brown sugar, cinnamon and chocolate chips until well blended. Set aside.
To make the muffins, in a large bowl, combine the bananas and sour cream. Add the cinnamon, ginger, salt, vanilla, egg and brown sugar. Combine until blended. Add the flour, baking powder, baking soda and butter and mix until just combined, Do not over mix. Fold in the choclate chips. Add a quarter of the streusel mixture and mix until just combined.
Using a 1/4 cup measure, fill the muffin cups and top each muffin with a spoonful of streusel, pressing lightly. Bake for 15-20 minutes, or until a toothpick comes out clean from center of muffin.