Mix 2.25 tsp instant yeast with 0.5 cup lukewarm milk and 1 tbsp sugar. Allow it to sit until frothy, about 5 minutes.

In a large bowl, combine 2.5 cups all-purpose flour, 2 tbsp potato starch (if using), and 0.25 tsp salt.

Add the 3 large egg yolks, softened 1 cup of unsalted butter, and the frothy yeast mixture to the dry ingredients. Knead until you have a soft dough.

Cover the dough and let it rise in a warm place until doubled in size, approximately 1 hour.

Roll the dough into a 9x9-inch square, cut it into rounds, and let them rise again for about 30 minutes.

Heat vegetable oil to 320°F (160°C). Fry the doughnuts for about 90 seconds on each side, or until golden brown.

In a saucepan, heat 1.5 cups of whole milk, 0.25 cup sugar, and vanilla extract until steaming.

In another bowl, whisk the yolks with 2 tbsp cornstarch. Gradually add this mixture to the hot milk, return it to the stove and cook until thickened. Add 1 tbsp unsalted butter. Set aside to cool completely.

Once your doughnuts have cooled, use a piping bag to fill each Bombolino with the pastry cream. Dust generously with powdered sugar before serving.