In a large mixing bowl, combine all-purpose flour, salt, and granulated sugar. In a separate bowl, warm the milk until it's slightly hot, then stir in fresh or instant dried yeast. Allow it to sit for about 5 minutes until it becomes foamy. Incorporate the melted butter and eggs into the flour mixture, followed by the foamy yeast mixture, and knead the dough for about 8 minutes until it’s smooth and elastic.

Transfer the kneaded dough into a greased bowl, covering it loosely with plastic wrap or a clean kitchen towel. Place it in a warm, draft-free spot to let it rise until doubled in size, approximately 90 minutes.

After the dough has risen, punch it down gently, then turn it onto a floured surface. Roll the dough out to about ½ inch thick. Using a round cutter, cut out circles and place them on a floured baking sheet. Allow the bomboloni to rest for 10 minutes.

In a deep skillet, heat vegetable oil to 350°F (175°C). Working in batches, lower each dough circle into the hot oil and fry for about 3-4 minutes until golden brown on both sides, turning them halfway through.

While frying, prepare the custard filling. Heat the milk over medium heat until steaming. In a bowl, whisk together egg yolks, sugar, and cornstarch. Gradually temper with the hot milk, return to the heat, and cook until thickened.

Once bomboloni have cooled slightly, use a piping bag to fill each pastry with custard filling. Dust with powdered sugar before serving.