In a large mixing bowl, combine 4 cups of all-purpose flour, 1/4 cup of granulated sugar, 1 tablespoon of instant yeast, and 1 teaspoon of salt. In a separate bowl, whisk together 1 1/2 cups of whole milk, 2 large eggs, and 1/4 cup of melted unsalted butter until smooth. Pour the wet ingredients into the dry mixture and stir to form a soft dough.

Transfer the dough to a floured surface and knead for about 10 minutes until it becomes smooth and elastic. Shape the dough into a ball and place it in a greased bowl, covering it with a damp cloth. Let it rise in a warm spot for approximately 1 hour, or until it has doubled in size.

In a medium saucepan, heat 2 cups of whole milk and 1/2 cup of granulated sugar over medium heat until just simmering. In a separate bowl, whisk together 4 egg yolks and 1/4 cup of cornstarch. Slowly add the hot milk mixture to the egg yolks while whisking constantly to prevent curdling. Return the mixture to the saucepan and cook on low heat, stirring until it thickens, about 5-7 minutes.

Remove the thickened pastry cream from the heat and stir in 1 teaspoon of vanilla extract and 3 tablespoons of unsalted butter until completely melted. Transfer the pastry cream to a bowl, cover it with plastic wrap, and refrigerate to cool for at least 30 minutes.

Once the dough has risen, punch it down gently and turn it out onto a lightly floured surface. Roll the dough out to about 1/2 inch thick, then use a round cutter to cut circles about 3 inches in diameter. Place the dough circles on parchment-lined baking sheets and cover them with a cloth. Allow them to rise for another 30 minutes.

In a deep pot, heat 2-3 inches of vegetable oil to 350°F (175°C). Carefully add a few dough circles at a time, frying for about 2-3 minutes on each side until golden brown. Use a slotted spoon to transfer the fried bomboloni to a paper towel-lined plate to drain excess oil.

Once the bomboloni have cooled slightly, fill a piping bag with the chilled pastry cream. Insert the tip of the piping bag into the side of each doughnut and gently squeeze to fill them with the cream until you feel a slight resistance.

Finally, dust the filled Bomboloni alla Crema generously with powdered sugar right before serving.