Mix the levain until thick, and leave it to rise until it reaches its peak.
Combine the eggs, milk, sugar, and all of the levain.
Gradually add the flour, butter, vegetable oil, salt, and vanilla extract.
Mix the dough until elastic, then place it on a lightly oiled surface.
Perform a few folds, shape it into a ball, and place it in an oiled bowl. Leave to rise until the volume increases.
Transfer the dough to a lightly oiled surface, divide it into 65–75 g pieces, roll them into balls, and place them on pre-prepared squares of parchment paper.
Cover with plastic wrap and leave to rise until the volume has tripled, at least 3 hours. Alternatively, place it in the fridge for a cold rise overnight.
Heat a generous amount of oil in a cast-iron pot or large deep pan to 180-185°C.
Transfer the bomboloni, still on the parchment paper, into the oil, gently remove the parchment with tongs, and fry for about 2.5 minutes on each side, flipping them over.
Remove to a plate lined with paper towels.
While still warm, roll the bomboloni in sugar and fill them with custard cream or any other cream/jam of your choice.
Another recipe:
https://www.tiktok.com/@preppykitchen/video/7348497340870724894