Melt the butter in a small pot on low heat.
Meanwhile, whisk the cinnamon and sugar together. Traditional ratio is around 1 part cinnamon to 3-4 parts sugar. Some people like this to be very sweet, and up to 16 parts sugar works. This also depends on where you're using the mix.
Brush melted butter on top of the donut.
Roll/toss the donut in the mix.