In a skillet set over medium heat, melt 6 tablespoons butter. Add brown sugar, maple syrup, and salt. Cook, stirring constantly for a few minutes until the mixture is smooth. If there are lumps in your sugar you might need to use a whisk. Once the mixture is smooth, remove from the heat and carefully stir in the bananas. Set aside and cool to room temperature while you prepare the French toast.

In a large bowl, whisk eggs, milk, half and half, brown sugar, cinnamon, and vanilla.

Grease a 2 ½ or 3 quart casserole dish and place half the bread cubes in the bottom. Spoon half of the banana-brown sugar mixture over the bread. Top with the remaining bread cubes.

Pour the custard (egg mixture) evenly over the bread cubes. Lightly press down on the top of the bread to allow the top layer of bread to absorb some of the custard mixture. Spoon the remaining banana-brown sugar mixture over the top of the French toast. Cover and refrigerate overnight. Prepare the topping.

Combine brown sugar, flour, cinnamon, and salt. Add butter, and using a spoon, work the butter into the dry ingredients until the butter is evenly distributed and the mixture resembles wet, clumpy sand. Cover and refrigerate until you are ready to bake the French toast.

Preheat oven to 350°F. Remove the French toast from the refrigerator (the bread will have absorbed most of the custard). Crumble the topping over the top of the French toast. Bake uncovered for 40-55 minutes, until puffed, golden brown, and set (check that the center is not too wet). Baking time will depend on how deep your casserole dish is and whether you prefer your French toast more well done. Serve immediately with syrup, caramel sauce, whipped cream and/or chopped nuts. Leftovers can be refrigerated.