When ready to bake, preheat the oven to 350°. Remove pan from the refrigerator and uncover. Combine 4 tablespoons butter, flours, brown sugar, cinnamon, and salt in a food processor. Pulse until the streusel topping looks granular. Sprinkle the topping over the bread. Bake until the custard is set and the topping is lightly browned, 30–35 minutes. Serve hot with maple syrup.