Using clean beaters, beat 2 egg whites until stiff peaks form. Gently fold into egg yolks. Beat butter until cream, gradually adding baking powder and optional vanilla. Mix in flour. Fold in egg mixture, stirring until a dough is formed.

Place dough into a zipper plastic bag and refrigerate overnight.

The following day, remove 1/4 of the dough and set aside. Roll the remaining dough out on a lightly floured surface or on silicone baking sheets. Press rolled out dough into buttered pan. Spread with guava paste (or other preserves). Use the remaining saved dough to either roll out strips for a lattice crust, or crumble it over the top for a crumb topping.

Bake in a preheated 350°F oven for 45 minutes, or until golden brown.