In a large mixing bowl, combine flour, salt, and granulated sugar. Create a well in the center and pour in warm milk and fresh yeast, letting it sit for 5 minutes until foamy.

Add softened butter and beaten eggs to the bowl, mixing until combined. Knead the dough for about 8 minutes until smooth and elastic.

Shape the dough into a ball and place it in a greased bowl, cover, and let it rise for approximately 90 minutes until doubled in size.

Punch down the dough, roll it out to ½-inch thickness, and cut out circles using a round cutter. Allow to rest for 10 minutes.

Heat vegetable oil in a pot to 350°F (175°C). Fry the bomboloni in batches until golden brown, about 2-3 minutes per side. Drain on a paper towel.

In a saucepan, heat milk until steaming. Whisk together egg yolks, sugar, and cornstarch, then gradually add hot milk while whisking. Cook until thickened.

Once cooled, fill each bombolone with custard using a piping bag and dust generously with powdered sugar before serving.