Stir streusel topping together in a medium bowl and set aside.  Combine flour, baking powder, baking soda, and salt in another medium bowl.  Using electric mixer, beat sugar, butter and egg in a large bowl until fluffy.  Beat in bananas and milk..  Add flour mixture and mix until just combined.

Spread half the batter into a greased 8x8-inch baking pan.  Sprinkle with half the streusel.  Repeat with remaining batter and streusel and bake in a 350 degree oven until toothpick comes out clean, about 45 minutes.  (I always check about five minutes early since oven cook times vary...this will prevent baked goods from becoming dry.)

Cool cake in pan on rack. This cake tastes best when fully cooled, even though it's tempting to dig right in...which is how I know this!