Preheat oven to 350 degrees F. Slice banana bread into slices and then cut into 1" cubes. Spread cubes in a single layer (as best as you can) onto a large baking sheet and bake for 4 minutes. Flip cubes with a spatula and return to oven for and additional 4 minutes.

Grease a 9 inch square baking pan with nonstick cooking spray and place half of the bread cubes into the pan. Top with sliced bananas. Add remaining bread to the top.

In a separate bowl, whisk together eggs, milk, half and half, sugar and vanilla. Gently pour on top of bread and use the back of a spoon to press the bread into the pan to soak up the egg mixture. Let stand for 5 minutes.

Bake for 40-50 minutes or until a knife inserted into the center comes out clean. Remove from oven and let stand to cool until warm. Serve warm topped with powdered sugar if desired.