Don't know about doughnuts, but potatoes fried in duck fat are beyond compare. Gross as it sounds, I collect and save the fat whenever I roast a duck (nothing fancy, store bought, prepared quite simply, only on rare occasion). I was taught by both French and Italian sides of the family to treat it like gold and use it sparingly. Sliced leftover new potatoes become golden, crisp and so-o-o savory fried in just a very little bit of this stuff.