Combine the first 4 ingredients in a small bowl, and stir with a whisk. Place 1/2 cup bread cubes into each of 2 (8-ounce) ramekins coated with cooking spray. Spoon 2 tablespoons milk mixture over each serving, and top with 1/4 cup banana. Sprinkle each serving with 1 1/2 teaspoons granulated sugar. Repeat procedure with remaining bread and milk mixture. Chill 30 minutes.
Preheat oven to 350°.
Bake at 350° for 50 minutes or until done. Spoon 1 tablespoon caramel sauce over each serving. Sprinkle with powdered sugar, if desired.