The plantains for this dish have to be very ripe, the peel almost black, but still have to be firm to the touch. When you press them gently, they should not feel mushy inside; if they are, the taste will be rancid and overripe. If you buy green plantains and wait for them to ripen it might take a week or so. Do not refrigerate them or they will not ripen well. It is better to buy plantains fully ripe or only a couple of days before, as this ensures you they haven't been refrigerated and will ripen fully without getting mushy.
Preheat the oven to 350° F. Cut the ends off the plantains and peel. Cut into 1/4-inch dice.
Pour the oil into a deep, medium, heavy pot, and place over medium-high heat. When the oil is hot (about 350° F), add the plantain cubes and fry for 2 to 21/2 minutes. Since they are small keep your eye on the frying pan all the time and swirl them in the oil with the slotted spoon. Remove the plantains from the oil with the slotted spoon and drain over paper towels; set aside.
In a small mixing bowl, combine the eggs, milk, butter, sugar, salt, vanilla, and cinnamon, if using.
In a medium bowl mix the fried plaintain cubes with the guava paste and the cheese.
Fold the egg mixture into the plantain mixture and pour into a greased and floured 8-inch square baking pan. Sprinkle extra cheese on top.
Bake 30 to 40 minutes until browned and bubbly.