In a large mixing bowl, combine lukewarm milk, instant yeast, and sugar. Let sit for 5 minutes until foamy.

Add flour, optional potato starch, and salt. Mix to combine.

Add egg yolks and softened butter. Knead until smooth and elastic—about 8–10 minutes by hand or 5 minutes in a stand mixer.

Cover the dough and let it rise in a warm, draft-free place for 1 to 1½ hours, or until doubled in size.

While the dough rises, make the pastry cream: heat milk in a saucepan until just steaming. In a bowl, whisk egg yolks, sugar, and cornstarch until pale and smooth.

Gradually whisk hot milk into the yolk mixture. Return to saucepan and cook over low heat, whisking constantly, until thickened (about 5 minutes). Remove from heat, stir in butter and vanilla. Chill with plastic wrap touching the surface.

Roll the dough out to ½ inch thick on a lightly floured surface. Cut into 2–3 inch rounds. Place on a tray, cover, and let rise again for 30 minutes.

Heat oil in a deep pot to 350°F (175°C). Fry bombolini 2–3 at a time, 1–2 minutes per side, until golden brown.

Drain on paper towels, then immediately roll in granulated sugar.

Once cooled slightly, fill each bombolone by piping chilled pastry cream into the center using a piping bag.

Serve fresh and enjoy every bite!