Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In large bowl, stir brown sugar, butter, corn syrup and walnuts until smooth. Gently stir in bananas. Spoon mixture into baking dish.
Reserve ends of bread for another use because they don't soak up the egg mixture very well. Arrange 2 layers of bread on banana mixture, tearing bread to fit if needed.
In medium bowl, beat eggs, milk and vanilla with wire whisk until well blended. Pour over bread. Cover tightly; refrigerate at least 1 hour but no longer than 24 hours.
Heat oven to 325°F. Uncover; bake 45 to 55 minutes or until knife inserted in center comes out clean. Serve portions upside down, spooning sauce from bottom of dish over each serving. Sprinkle with powdered sugar.