1/4 cup fresh compressed yeast (or 1 1/2 pkgs. active dry)
1/4 cup warm water (105F - 110F)
3 1/2 cups bread flour
4 large eggs
1/3 cup granulated sugar, plus extra for coating
1 teaspoon kosher salt
14 tablespoons unsalted butter, cubed
as needed -- veggie oil, for deep-frying
as needed -- pastry cream, jam, preserves or other filling
Dissolve the yeast and water in a small bowl and let stand until foamy, about 10 minutes.
Combine flour, eggs, sugar, and salt in the bowl of a stand mixer fitted with the paddle attachment and mix just until combined. Add the bloomed yeast and mix for about 2 minutes on medium-high speed, until the dough is well combined and holds together. Add the cubed butter and mix for 5 minutes, until the dough no longer sticks to the side of the mixing bowl. Be sure to stop the mixer occasionally and scrape down the bowl with a rubber spatula.
Remove the paddle, pat the dough down into a ball, cover the mixing bowl with plastic wrap and let the dough rest and proof slightly at room temperature for 30 minutes.
Remove the dough from the mixing bowl and flatten it down to remove the air. Spread it onto a lightly floured baking sheet with your fingers and flatten to about 1 inch thick. Cover with plastic wrap and let it rest in the refrigerator overnight.
Remove the dough from the refrigerator and place on a lightly floured work surface. Flatten it to about 1/2 inch thick with your hands. Cut the dough into circles with a 1 1/2-inch diameter cutter. Continue to flatten and cut remaining dough into circles until all the dough is used up.
Arrange the circles on a parchment or Silpat covered baking sheet lightly sprayed with vegetable cooking spray. Space them out to allow room for the donuts to proof and rise. Loosely cover with plastic wrap and proof at room temperature until they have doubled in size and appear light and full of air, about 2 hours.
To cook, heat veggie oil to 325 degrees F. Fry a few donuts at a time, being careful not to overcrowd the pan. Fry until they are golden brown, flipping once the first side has browned. Remove the donuts with a large slotted spoon and set on a paper towel to drain the excess oil. While the Bomboloni are still warm, roll them in a bowl filled with granulated sugar. Enjoy as is, or fill the Bomboloni with a pastry bag fitted with a small round tip. Use a sharp paring knife to make a small hole on the bottom of each Bomboloni, then gently fill with your favorite filling. It is best to fill the Bomboloni while they are still warm. Serve.