I used brioche which is a soft, slightly sweet bread. Challah is a great sub since it's also soft, rich, and slightly sweet. You could probably use croissants, too. Sourdough, or diced French bread from a baguette or loaf are also acceptable, although not as sweet, not as rich, and drier. However, don't just use regular sliced bread (no white sandwich bread!) that you'd make sandwiches with because it's not the right texture or flavor for this dessert-like recipe.