Prepare the Dough: In a small bowl, combine the warm water, yeast, and 1 tablespoon of sugar. Let it sit for about 5 minutes, until it becomes frothy.

Make the Dough: In a large mixing bowl, combine the flour, sugar, salt, and lemon zest. Add the eggs, milk, melted butter, vanilla extract, and the yeast mixture. Mix until a dough forms. Knead the dough on a floured surface for 8-10 minutes until smooth and elastic.

Let the Dough Rise: Place the dough in a lightly greased bowl, cover with a damp cloth, and let it rise for about 1-1.5 hours, or until it doubles in size.

Shape the Donuts: Punch down the dough and turn it out onto a floured surface. Roll it out to about 1/2-inch thickness. Using a round cutter (about 3 inches in diameter), cut out circles of dough. Place them on a floured baking sheet and cover with a cloth. Let them rise for another 30-45 minutes, until puffed up.

Fry the Donuts: Heat vegetable oil in a deep fryer or large pot to 350°F (175°C). Carefully drop the donuts into the hot oil, a few at a time, and fry for 2-3 minutes per side, until golden brown. Use a slotted spoon to transfer the donuts to a paper towel-lined plate to drain excess oil.

Coat the Donuts: While the donuts are still warm, roll them in a mixture of sugar and cinnamon (if using).

Fill the Donuts: Let the donuts cool slightly. Using a pastry bag fitted with a small tip, inject the pastry cream into the center of each donut.

Serve: Serve the Bomboloni fresh and enjoy the delicious cream-filled goodness!