Shape the doughnuts: Turn the dough out onto a well floured surface. Weigh the dough, and divide into 14 equal weight portions; each piece of dough should weigh about 75g. Roll each portion into a tight ball and then slightly flatten the ball into a disc shape. Place the doughnuts on a baking sheet lined with parchment paper, leaving an adequate space between each one, then cover with plastic wrap. (You will need two baking sheets.)
Allow the doughnuts to proof for 20 minutes (this will take longer if your dough is cold but just check after 20 minutes to see how they are doing). When you poke them lightly with your finger, it should leave a small indentation.
While the doughnuts are proofing, heat the canola oil to 350°F (180°C) in a deep fryer or a large heavy-bottomed pot (cast iron works great). Line a baking sheet with a double layer of paper towels for draining the doughnuts. Place some granulated sugar in a small bowl and set aside.
Fry the doughnuts: Once the oil has come to temperature, gently lower the doughnuts (no more than 2-3 at a time) into the hot oil. Fry until golden brown on the underside, 2 to 3 minutes. Carefully turn the doughnuts over and fry for an additional 2 to 3 minutes, or until the other side is golden brown as well. Using a slotted spoon, remove the doughnuts to the paper towel-lined baking sheet to remove any excess oil, and then transfer to a cooling rack. Repeat the process with the rest of the doughnuts.
As soon as the doughnuts are cool enough to handle, toss them in the sugar one at a time to evenly coat them. Return the doughnuts to the cooling rack and allow to cool completely, about 30 minutes. In the meantime, transfer the cooled pastry cream and apple pie fillings to their own piping bags.
Poke a hole in the side of each doughnut and fill with the apple pie filling. Then fill with the pastry cream. The doughnuts should be served as soon as they are filled. They are best the same day they are made.