Make Dough: Start by activating the yeast with warm milk and sugar. Once it's bubbly, mix the flour, potato starch (if using), and salt in a bowl. Add the egg yolks and softened butter. Knead everything into a soft dough and let it rise until it doubles in size.

Shape Bombolini: Roll the dough into a 9x9 inch square. Cut out circles and place them on parchment paper. Cover them and let them rise again until they get puffy.

Fry: Heat oil in a pot to 320°F (160°C). Fry the donuts for 90 seconds on each side until they are golden brown. Let them drain on paper towels and then roll them in sugar.

Pastry Cream: In a saucepan, heat the milk, sugar, and vanilla. In a bowl, mix the egg yolks with a little of the hot milk to temper them. Whisk this back into the saucepan. Cook the mixture until it thickens, then let it cool completely.

Fill: Once the donuts are cooled, poke a hole in each one. Pipe in the pastry cream and dust with powdered sugar. Enjoy!