*Nedd uses Bakewell Cream in place of cream of tartar. She says that it gives her donuts more springiness.
In a bowl, combine the eggs, sugar, squash, buttermilk, butter and vanilla. Combine the dry ingredients; add to the squash mixture and mix well. Cover and refrigerate for 2 hours (dough will be very soft).
Turn dough onto a heavily floured surface; roll to ½-inch thickness. Cut with a 2½-inch donut cutter. Heat oil to 375°. Fry the donuts, a few at a time, until golden brown on both sides. Drain on paper towels. Toss with sugar, if desired.