From my many tests, I have found that the best temperature is about 170 degree Celsius (340 degree Fahrenheit). Because it contains a lot of eggs, the donuts tend to brown quickly. If the temperature is too high (over 180 degree Celsius or 350 degree Fahrenheit), the donuts will burn before they are fully cooked and their centre will be raw. Under 165 degree Celsius (330 degree Fahrenheit), the donuts will fry to slowly resulting in overly greasy donuts. Make sure to keep your thermometer in the frying oil and keep playing with the temperature of your stove when required to insure the oil stays around the recommended temperature.