Build the levain using a ripe sourdough starter and store at around 78°F (25°C) until ripe, about 5 hours.
Mix the dough ingredients in a stand mixer, starting with the flour, sugar, eggs, levain, salt, and water. Mix until combined, then add softened butter and mix until the dough almost passes the windowpane test.
Transfer the dough to a container for warm bulk fermentation at 78°F (25°C) for 6 hours, performing 4 sets of stretch and folds during this time.
After bulk fermentation, place the dough in the fridge for cold bulk fermentation overnight.
Divide the dough into eighteen 65g pieces and shape them into rounds, ensuring a taut surface.
Proof the shaped rounds in a warm spot for about 12 hours until they are puffy and delicate.
Fry the proofed dough in oil at 360-370°F (182-187°C) for 2.5 minutes per side until golden brown.
Toss the fried bomboloni in a mixture of sugar and cinnamon while still warm.
