In a large mixing bowl, combine flour, salt, and sugar, creating a well in the center. Dissolve yeast in warm milk, pour into the well, and let sit until foamy, about 5 to 10 minutes.

Add softened butter and beaten eggs, then knead until smooth and elastic, about 8 minutes.

Transfer the kneaded dough to a greased bowl, cover with a damp cloth, and let rise in a warm area until doubled in size, about 90 minutes.

Once risen, punch down the dough, roll out to ½ inch thickness, and cut into circles, placing them on a floured baking sheet to rest for 10 minutes.

Heat oil to 350°F (175°C) in a deep pot or fryer, carefully place bomboloni into the hot oil, frying for 3 to 4 minutes until golden brown.

In a saucepan, heat milk and whisk together yolks, sugar, and cornstarch until smooth. Gradually combine with hot milk, then return to the pan and cook until thickened, around 5 minutes.

Remove custard from heat, whisk in vanilla, and cool completely.

Transfer cooled custard to a piping bag, fill each bombolone, and dust with powdered sugar before serving.