The cinnamon streusel topping adds a delightful crunch and an extra layer of flavor to your French Toast Casserole. To create the perfect streusel, combine flour, brown sugar, cinnamon, and cold butter in a bowl. Use your fingertips to rub the butter into the dry ingredients until crumbly. Sprinkle the streusel over the soaked bread just before baking to achieve a golden, caramelized topping that will make your taste buds sing.