In a stand mixer fitted with the paddle attachment, add lukewarm milk then sprinkle dry yeast on top and leave it aside to activate. In 5 minutes, add sugar, eggs, salt, flour, and butter. Place your dough in your mixer and knead the ingredients until it is firm and smooth. Using your hands, shape it into a ball. Place it in a lightly greased bowl covered with plastic film. Let the dough rest in warm spot for an hour and fifteen minutes or until it doubles in size.
To make the vanilla pastry cream, add half of the milk and sugar to a pot, then bring to a simmer over medium-high heat. In a small bowl, place corn starch, egg yolks, vanilla extract and the rest of the milk and the whipping cream. Mix well over low heat until thickened. Then add unsalted butter into the pastry cream and mix well. Keep it refrigerated until the pastry cream is completely chilled.
Divide the dough into 12 even pieces. Form each piece into balls by using the palm of your hand and rolling until a compact ball is formed. Place your donuts and cover them with a dish towel, until they double in size. Place the frying oil into a large pot and heat it to 350F (180C). Fry the Bombolini for 2 minutes on each side.
Remove the Bombolini from the oil when they turn golden brown in color. Place them on a baking rack to cool for 2 minutes then proceed to roll the Bombolini with the cinnamon sugar mix. Finally, fill each Bombolini with the chilled vanilla pastry cream. Make a small hole on the side of each donut and inject the pastry cream into each donut.
