1. Preheat oven to 350 degrees F. Line 9x13 baking pan with parchment paper, then lightly spray paper with cooking oil.
2. Combine brown sugar, flour, baking soda, salt, and rolled oats in a large bowl.
3. Cut butter into dry mixture with a pastry blender until mixture is a crumb-like texture.
4. Press half this mixture into prepared baking pan.
5. Slice guava paste evenly to cover the crust, putting within 1/4 inch of the edge of crust in baking pan.
6. Sprinkle remaining crust mix over guava paste layer, then press down lightly with the back of a spatula or fork.
7. Bake in preheated oven for 35 to 40 minutes, until crust is light brown.
8. Cool in pan on a wire rack before cutting into bars. Yields from 16 to 24 bars, depending on size cut.
Guava paste is the surprise ingredient in this basic recipe. Cut these up and just pile them on a plate, then watch the surprise when your family digs in. Delicious!
If you want these bars to be more dessert-like, make a powdered sugar frosting and drizzle it on lightly in strings and let it harden before cutting and serving.