Combine all-purpose flour, salt, and granulated sugar in a mixing bowl. Create a well in the center and pour in warm milk mixed with yeast; let it sit until frothy and bubbly, about 5-10 minutes. Add sweet butter and eggs; knead the dough on a floured surface for about 8 minutes until smooth and elastic.

Place kneaded dough in a greased bowl, cover with a kitchen towel, and let rise in a warm environment for about 90 minutes or until doubled in size.

Once risen, punch the dough down gently, roll it out to about ½ inch thick, cut out circles with a cookie cutter, and place them on a lined tray. Cover and let rest for 10 minutes.

Heat vegetable oil to 350°F (175°C) in a deep pot. Fry the dough circles in small batches for 3-4 minutes until golden brown, using a slotted spoon to remove and drain on paper towels.

Heat whole milk in a saucepan. In a separate bowl, whisk together egg yolks, sugar, and cornstarch until smooth. Gradually whisk in warm milk, return to the saucepan, and cook over medium heat, stirring until thickened. Stir in vanilla extract and cool completely.

Using a piping bag fitted with a star tip, fill each cooled bomboloni with custard. Dust generously with powdered sugar before serving.