1(16.3 oz) can of biscuit dough—it should make 8 biscuits (I used Grands southern-style)
Pour one inch of vegetable oil into a heavy pan (like a cast iron pan) or fryer. Heat the oil to 360°F–375°F.
Open the can of biscuit dough and separate the dough into 8 biscuits. Use a small cutter to cut holes out of the center of each biscuit (I used a frosting coupler). Alternatively, just use your finger to make a hole in the middle.
If you are making doughnut holes, cut each piece of dough into 4 pieces, and roll the pieces into balls. You should have 32 balls total.
Carefully slide the dough (either the 2–3 whole doughnuts or 6–8 doughnut holes) into the hot oil. Fry until golden brown, then flip and fry until golden brown on the other side. This will take about 2–3 minutes per side, depending on how hot the oil is. Repeat with the remaining dough.
Meanwhile, place the sugar and cinnamon (if using) in a paper bag.
When the doughnuts are cooked, carefully remove them from the oil and place on a paper towel lined plate. Let sit for 30 seconds to drain any excess oil, then put the doughnuts into the bag with the sugar and cinnamon. Fold the bag to close, and gently shake the bag to distribute the sugar evenly onto the doughnuts. You may need to do this in batches.
Remove the doughnuts from the bag and serve immediately. You can also keep them warm on a sheet pan in a 200°F oven for up to 20 minutes.
Reheating instructions: Heat leftover doughnuts in a preheated 350°F oven for approximately 5 minutes, or until warm. Do not microwave the doughnuts to reheat them (they will get tough and chewy).