In a bowl of a standing mixer, pour the milk, yeast, and one tablespoon of sugar. Let sit for 15 minutes to activate the yeast.
In a separate large bowl, mix the flour, potato starch, rest of the sugar, and salt.
Add the dry ingredients to the yeast mixture following the egg yolks, and melted butter.
Use the paddle attachment to hydrate the ingredients, once you have a shaggy mass, replace the attachment with the dough hook.
Mix on medium speed for about 3 minutes. The dough should be soft and stretch without tearing when pulling it gently.
Spray the top of the dough with oil spray, cover with plastic wrap, and let rise until double in size (about 45-60 minutes)
Punch the dough, and transfer it to a lightly floured surface.
Roll the dough to a 9x9 square and use a 2-inch or a 3-inch cookie cutter to cut the dough.
Place each piece of dough over a 4x4 square of parchment paper then over a large baking sheet. Cover with plastic wrap and let rise for about 60 minutes.
warm the oil to 320F (160C) in a wide and deep pan.
Gently drop a few donuts into the hot oil, and fry for about 3 minutes on each side. Use kitchen tongues to remove the parchment paper.
Use a slotted spoon to remove the donuts and place them over a place lined with paper towels.
Allow the donuts to cool for about 10-15 minutes before filling.
Coat the donuts with powdered sugar or granulated sugar.
Fill a pastry bag with the pastry cream.
Use a sharp knife to make an insertion at the side of each donut. Fill the donuts with the filling. Serve.