Prepare your frying set up.
Place a large, heavy pot on the stove and clip a deepfry/candy thermometer to the side of the pot with the probe pointing to the middle.
Pour 2-3 inches of oil in and set the pot over medium heat. I like to use canola, because of its high smoke point and it doesn't effect the taste. My absolute favourite for frying is lard, but you can use any oil with a high smoke point.
The goal is to get the oil to reach 375°F and it is important to watch the pot at all times to maintain that temperature.
If you have a deep fryer, you're a lucky devil, just set it to 375°.
Next to the stove or stand alone deep fryer, set up a cooling rack over a cookie sheet covered with a tea towel or a paper towel to catch drips and make clean up easier. It's best to set the fresh fried doughnuts onto a rack instead of directly onto a towel because the towel will suck up any excess oil but the doughnut will then be sitting in it, and you'll get the dreaded Soggy Bottom.
Make sure to have your preferred tools for flipping and remove the doughnuts from the oil ready. Tongs, a spider, chopsticks, or a sturdy, all metal strainer will all work.
Fry at 375 degrees. I fry most doughnuts at around the 360 mark, but I like to keep the oil hotter for cake doughnuts because they are heartier and can take the heat without burning. More importantly, it keeps the time they are floating in the oil to a minimum, so their cakey texture doesn't soak up any grease.
** If you find yourself frying without a thermometer, you'll have to be extra observant. Keep a scrap piece of dough or an ill formed doughnut hole and throw it in as a tester. It should bubble and fizz as soon as it hits the oil, but and be cooked on that side in 10-20 seconds for a small piece of dough. If it burns in that time, turn off your heat and wait 5-10 minutes for the oil to cool down. Generally if your oil is hot enough to do that, it's already starting to smoke anyway, so you'll know to turn it down.**
Once the oil has reached 375°F, begin to add the doughnuts, one at a time by sliding them carefully into the oil. Don't drop them in or you'll have screaming hot oil splash all over you.
Cake doughnuts will sink to the bottom of the oil, then cook for a bit before rising up. Once the doughnut bobs to the surface, fry it for approximately 1 minute more before turning it with your chopsticks or tongs and fry for an additional 1-2 minutes on the other side. Only fry 3-4 at a time, overcrowding the pot will cause the temperature to drop and the doughnuts to suck grease.
Place the doughnuts on the prepared cooling rack. Break one open to ensure that it is cooked, and if additional time is needed, add the doughnuts back to the oil to finish and add a bit more cooking time for the rest. Fry the rest in similar fashion, and allow all of the doughnuts to cool.
If you've overworked your dough, you will see the pale tops of the doughnut split and crack open-- that's okay, just flip it at that point and finish frying. They will taste just as good when youre done, just with a tighter crumb and a crunchy top that looks oh so rustique.
If youve rolled your doughnuts thicker than 3/4" they will take longer to fry. Pull one out when you think it should be done, wait 5 minutes for it to finish off cooking and become cool enough to handle. Break it open. If its still gooey inside, keep the next ones in 20-30 seconds longer.