1. Place butter in a microwave-safe bowl; microwave, covered, until melted, 30-45 seconds. Stir in brown sugar, cream, cinnamon, allspice and, if desired, pecans. Add bananas; toss gently to coat.
2. Transfer to a greased 13x9-in. baking dish. Arrange bread over top, trimming to fit as necessary.
3. Place remaining ingredients in a blender; process just until blended. Pour over bread. Refrigerate, covered, 8 hours or overnight.
4. Preheat oven to 375°. Remove French toast from refrigerator while oven heats. Bake, uncovered, until a knife inserted in the center comes out clean, 35-40 minutes. Let stand 5-10 minutes.