Prepare the Dough: In a bowl, mix lukewarm milk, sugar, and active dry yeast; let it sit until frothy, about 5-10 minutes. Combine with eggs and melted butter, then add flour and salt. Knead until smooth, then place in a greased bowl to rise until doubled, around 1-2 hours.

Make the Custard Filling: Whisk together sugar, cornstarch, and salt in a saucepan. Gradually add milk while stirring. Heat over medium, thickening until creamy. Temper in the egg yolks and combine; cover with plastic wrap on the surface to cool.

Shape the Bomboloni: Roll out risen dough to about ½ inch thick. Cut circles about 3 inches in diameter and place on a floured tray to rise for another 30 minutes until slightly puffy.

Fry and Fill: Heat oil in a deep pot to 350°F (175°C). Fry the Bomboloni for 2-3 minutes on each side until golden brown. Drain on paper towels and fill with custard using a piping bag. Finish by dusting with powdered sugar before serving.