Activate the Yeast:

• In a small bowl, combine lukewarm milk, instant yeast, and sugar. Stir to dissolve and let sit for 5 minutes until it becomes foamy.

Make the Dough:

• In a large bowl, mix the flour, potato starch (if using), and salt. Create a well in the center and add the yeast mixture along with the egg yolks. Mix until the dough begins to come together.

Knead the Dough:

• Add the softened butter to the dough and knead for about 10 minutes (either by hand or using a stand mixer). The dough should be smooth, elastic, and slightly sticky. If it’s too sticky, add a little flour, but be careful not to overdo it.

Let the Dough Rise:

• Form the dough into a ball and place it in a lightly oiled bowl. Cover with a clean towel and let it rise for 1 to 1.5 hours, until it has doubled in size.

Shape the Bombolini:

• Once the dough has risen, punch it down to release the air. Roll the dough out on a lightly floured surface to about ½ inch thick. Use a round cutter (2-3 inches in diameter) to cut out circles of dough.

Let the Dough Rise Again:

• Place the dough circles on a parchment-lined baking sheet. Cover with a towel and let them rise for another 30 minutes, until they puff up slightly.

Fry the Bombolini:

• Heat oil in a large pot to 350°F (175°C). Fry the Bombolini in batches, ensuring not to overcrowd the pan. Fry for 2 minutes per side, or until golden brown. Remove with a slotted spoon and place on paper towels to drain excess oil.

Coat and Serve:

• While still warm, roll the Bombolini in granulated sugar. If desired, fill them with your favorite jam, custard, or cream before serving.