Heat oven to 350°F. Brush a 9x13-inch baking dish with 1 tbsp melted butter.
Spread the bread cubes in the dish. Drizzle with 1 tbsp melted butter and toss to coat.
Whisk the eggs, milk, cream, mashed bananas, brown sugar, granulated sugar, vanilla, cinnamon, nutmeg, and salt until smooth.
Pour the custard over the bread. Gently press so all cubes soak evenly. Rest 10 minutes.
Sprinkle nuts on top if using. Drizzle remaining 1 tbsp melted butter over the surface.
Bake 35–40 minutes, until puffed, golden, and the center is set.
Cool 10 minutes. Slice and serve warm with maple syrup.