In a large mixing bowl, combine the all-purpose flour, salt, and granulated sugar. Create a well in the center and pour in warm milk, then sprinkle the fresh yeast on top. Allow the mixture to sit for 5 minutes until it foams.

Add melted butter and beaten eggs to the mixture, then knead the dough for about 8 minutes until it's smooth and elastic.

Form the dough into a ball and place it into a greased bowl, turning to coat. Cover with a clean kitchen towel and let it rise in a warm area for approximately 90 minutes, or until it has doubled in size.

Punch down the dough gently to release air. Roll out to about ½ inch thickness on a floured surface. Cut out circles with a round cutter and place them on a lined baking tray. Cover and let rest for an additional 10 minutes.

Heat vegetable oil to 350°F (175°C). Carefully add dough rounds in batches and fry for 2-3 minutes on each side until golden brown. Drain on paper towels.

Heat whole milk in a saucepan until steaming. In another bowl, whisk egg yolks, granulated sugar, and cornstarch until smooth. Gradually add hot milk while whisking, then return to saucepan and cook until thickened. Stir in vanilla extract and let cool.

Once bomboloni are cool, make a small slit in each and fill with custard using a piping bag. Dust with powdered sugar before serving.