Coat the donuts. While the donuts are still a little warm, gently melt the vegan butter in a microwave or in a pan over a very low heat on the stovetop. In a bowl big enough to fit the donuts into for dipping, combine the sugar and cinnamon. Using the butter ensures a good amount of sugar sticks to the donuts and helps give a “fried” flavor. However, if you want to keep the recipe oil-free, you can omit the butter, dipping the donuts into the cinnamon-sugar mixture while they are hot, and it will stick to them quite well.