Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch baking pan.
In a medium bowl, sift together the flour, baking soda, baking powder and salt. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the guava and milk. Stir in the walnuts if desired. Spread evenly into the prepared pan.
Bake for 30 to 40 minutes in the preheated oven, or until a toothpick inserted into the center of the cake comes out clean. Let cool in pan.
Spread cream cheese frosting over the cooled cake.
Interesting Facts
Guava is a tropical fruit native to the Caribbean and Central America.
Guava is high in Vitamin C and a good source of dietary fiber.
Cream cheese frosting is a classic topping for cakes and cupcakes.