Make the shells: In a processor, blend the flour, sugar, salt, butter and shortening until mixture resembles meal; transfer to a bowl and add 2-3 tblsps of ice water, or enough to form dough; knead dough briefly on a floured surface and chill it, wrapped plastic wrap, for 1 hour; roll out dough 3/8 inch thick on a floured surface, cut into rounds with a 4 1/2 inch cutter, and fit the rounds into 1/3 cup muffin tins.