Preheat oven to 325F.
In the bowl of a standing mixer, combine the sugar and egg yolks. Using the paddle attachment, beat the eggs until they've lightened. Add the melted butter, vanilla extract, potato starch and salt. Mix until all of the lumps have disappeared and the mixture is smooth. Slowly add the buttermilk and whole milk and mix well to incorporate. (You may have to scrape the sides of the bowl to fully incorporate.)
Place the egg whites and a pinch of salt in the cleaned bowl of the electric mixer. Using the whip attachment on medium-high speed, beat the egg whites they form firm peaks. Fold the egg whites into the batter quickly and lightly.
Grease ten ramekins with butter and place in a hotel pan. Fill the pan half way with water. Bake until golden brown, 20-30 minutes.
After the budino has baked, chill for 2-4 hours or overnight. Serve with whipped cream, berries, toasted nuts, or gelato (or all of the above).