You can make the dough the night before. Instead of letting it rise in a warm place for the first rise, place the covered bowl in the refrigerator for up to 12 hours. Then continue as directed in the recipe.
Try to keep the oil temperature as constant as possible. Too cool, and the doughnuts will absorb more oil and take longer to cook. Too hot, and the doughnuts will get done on the outside too quickly, leaving the centers still doughy. It will take about 20 minutes to heat the oil to 325°F.
These doughnuts can be filled with a variety of fillings. Try lime curd or any flavor of jelly.