The next day, pull out your donut dough out of the fridge and cut it into 53g pieces. This will make about 10 donuts.
Roll each 53g piece of dough into a perfectly tight little bun. Place the buns onto a floured surface, making sure to leave enough room between them because they will get a lot larger and we don't want them to stick together. Cover the buns loosely and let them rest for 4 hours or until they double in size.
Once the donuts are almost ready to be fried, fill a deep fryer halfway with neutral flavored oil and heat to 345F.
When the oil reaches the right temperature, carefully pick up the donuts, making sure not to deflate them and slide them in the hot oil. Fry for about 2 minutes on each side. Depending on the size of your fryer you can fry 2-4 donuts at the same time without dropping the temperature too much.
Remove the donuts from the hot oil and place them on kitchen paper to get rid of any excess oil. Once they are cool enough to handle but not so cool that the sugar doesn't stick, toss them in a bowl of granulated sugar. Place them on a cooling wrack to let them cool completely before filling.
To fill the donuts, make a whole using chopsticks of a small knife on the side of the donut. This will be on the lighter line that develops between the fried top and bottom.
Fill up a piping bag that has a medium sized piping tip at the bottom of it with apple filling and gently fill the donuts in the whole you made previously. Fill the donuts until they are a perfectly swollen.
These donuts are best served right away! You can store them in an airtight container for later but they will not be the same.