In a large mixing bowl, whisk together all-purpose flour, salt, and granulated sugar until well combined. Create a well in the center and add warm whole milk mixed with fresh or instant dried yeast. Let this mixture sit for 5 to 10 minutes until it's foamy. Then, mix in softened sweet butter and beaten eggs, kneading the dough for about 8 minutes until smooth and elastic.

Once your dough is kneaded, place it in a greased bowl, covering it with a damp cloth. Allow the dough to rise in a warm area for approximately 90 minutes or until it has doubled in size.

After the dough has risen, gently punch it down to release the air. Roll out the dough on a lightly floured surface to about ½ inch thick. Using a round cutter, cut out circles of dough and transfer them to a lined tray. Let the cut-out circles rest for another 10 minutes to become slightly puffy.

In a deep frying pan, heat vegetable oil to 350°F (175°C). Carefully place each dough circle into the hot oil and fry for about 3 to 4 minutes on each side, or until they are golden brown and puffed up.

In a medium saucepan, heat whole milk over medium heat until steaming—but do not boil. In a separate bowl, whisk together egg yolks, sugar, and cornstarch until smooth. Gradually add the hot milk to this mixture while whisking continuously to avoid curdling. Return the mixture to the heat and cook until it thickens, stirring frequently, then remove from heat and stir in vanilla extract. Allow the custard to cool completely.

Using a piping bag fitted with a long nozzle, carefully fill each bombolone with the rich custard filling. Dust them with powdered sugar before serving to enhance flavor.