In a saucepan, melt butter on low heat. Add marshmallows and stir until melted. (You can also microwave the butter and marshmallows for 1 minute on HIGH, if you prefer)
Remove from heat. Add guava marmalade and mix until the marshmallows turn into a pinkish red colored cream. Immediately mix in the rice cereal, cubes of guava paste, and crumbled Maria cookies. Stir until all is well combined.
Spray a spatula with cooking spray and transfer the mixture into the pan, pressing down evenly. Let cool. Cut into 9 or 12 squares and serve.