Lightly grease a 9x13 inch baking dish.
Spray the baking dish lightly with nonstick cooking spray. Scatter cubed butter over the bottom of the dish. Arrange bread slices in rows in the pan, with slices slightly overlapping. Slide banana slices between the slices of bread.
In a medium mixing bowl, whisk the eggs until well combined. Whisk in the milk, heavy cream, granulated sugar, vanilla extract, cinnamon, and ¼ teaspoon salt. Pour the liquids over the bread.
Cover with foil and refrigerate for 2 hours to overnight.
About 20 minutes before baking, remove the pan from the refrigerator and preheat the oven to 350°F / 177°C.
Place the casserole dish on a sheet pan to bake (this will help catch any drips that might bubble over as it cooks). Bake the French toast, covered with foil, for 40 minutes.
While French toast bakes, blend together 4 Tablespoons chilled butter, brown sugar, and ¼ teaspoon salt with a pastry cutter or with your fingers. Add pecans and blend them in until evenly combined.
When French toast has been baking for 40 minutes, remove it from the oven and remove foil. Scatter pecan crumble over top.
Return pan to oven and continue baking until the center is set and the pecans are lightly toasted, 10 to 15 minutes more.
Slice and serve with a dusting of powdered sugar and / or syrup.