I've seen both, but auto-correct approves only "Doughnut".
Most Helpful Girl
The official dictionary spelling of the word is doughnut with donut generally being listed as a variant of the preferred original spelling. Doughnut is the original spelling of the word, coming onto the scene in the early 1800s.0
Most Helpful Guy
Doughnut is the original spelling of the word, with donut being a more modern alternative spelling. Both are commonly accepted, yet doughnut is more correct.1