Butter provides a creamier touch, although, some prefer to substitute it with margarine.
If you prefer to go vegan, just replace the regular cow's milk with any nut milk.
If you prefer to go sugar-free, just replace white sugar with maple syrup, honey or coconut sugar. The sweetener you use should be able to caramelize the milk. So, you cannot use Splenda, stevia or xylitol.