Preparing the Dough

1. In a large bowl, mix flour, sugar, yeast, and salt.
2. Gradually add warm milk, eggs, and melted butter while kneading.
3. Knead until smooth and elastic, about 10 minutes.
4. Cover the bowl and let the dough rise in a warm place for 1–2 hours until doubled in size.

Shaping and Proofing

1. Roll the dough out to ½-inch thickness and cut into circles using a cookie cutter.
2. Place the circles on a lined baking sheet, cover, and let proof for 30 minutes.

Frying the Bomboloni

1. Heat oil in a deep pot to 350°F (175°C).
2. Fry dough circles in batches, flipping halfway, until golden brown.
3. Drain on paper towels.

Making the Custard Filling

1. Heat milk with vanilla in a saucepan over medium heat.
2. Whisk egg yolks, sugar, and cornstarch in a separate bowl.
3. Gradually pour the hot milk into the egg mixture, whisking constantly.
4. Return the mixture to the pan and cook over low heat until thickened.

Filling and Coating

1. Cool the Bomboloni before filling.
2. Use a piping bag with a small tip to inject custard into the center of each donut.
3. Roll filled donuts in granulated sugar for a sweet finish.