In a large mixing bowl, combine all-purpose flour, salt, and granulated sugar. In a separate bowl, warm the whole milk and blend in the fresh yeast. Let sit for 5-10 minutes until foamy. Incorporate the yeast mixture into the dry ingredients, then add softened butter and beaten eggs, mixing until a sticky dough forms.

Transfer the dough onto a floured surface and knead for 8 minutes until smooth and elastic. Form into a ball, place in a greased bowl, cover with a damp cloth, and let it rise for 90 minutes or until doubled in size.

After the first rise, punch down the dough to release air. Roll it out to about ½ inch thick and cut out circles using a round cutter. Place on a floured tray and let rest for 10 minutes.

Heat vegetable oil in a deep pot or fryer to 350°F (175°C). Fry dough rounds in small batches for 2-3 minutes on each side until golden brown. Remove and drain on paper towels.

In a saucepan, heat whole milk over medium heat until simmering. In a bowl, whisk egg yolks, sugar, and cornstarch until smooth. Gradually add half the hot milk to temper the egg mixture, then return it all to the saucepan, stirring until thickened, about 5-7 minutes. Remove from heat and stir in vanilla extract. Let cool.

Slice a slit into each cooled doughnut and use a piping bag to fill each bombolone with custard. Dust with powdered sugar before serving either warm or at room temperature.