Peel and core guavas. Rub cores through sieve to obtain some of the pulp, and mix with cut up guavas. Add sugar to taste.
Beat butter, Crisco with sugar, then beat in egg yokes.
Sieve flour, baking powder and salt and fold into butter/egg mixture. Add vanilla.
Divide dough in 3 balls. Refrigerate 1/2 hour until firm.
Spread out each of the balls on a piece of foil; cover with Guava. Starting at one end, fold into roll.
Cover with foil, place in baking bag and seal. Boil 1 hour in large pot with hot water. Slice and serve warn with Sauce.
FOR SAUCE: Beat egg whites in dish until foamy. Cream butter and sugar in a separate dish until sugar has dissolved. Gradually add egg whites and vanilla and continue beating till smooth. Add to top of warn Guava Duff when serving.