Betta's will devour any fry they can get in their mouth. if it is alive and moving and small enough to fit, it is fair game to them!
I like MrVamp's idea, make a paste food. chop up blood worms, daphnia, grind up some betta pellets, black worms. just be sure if the food is love before you start that you rinse it well.
But the best treat in the betta's eyes is live food, be it worms (do not feed tubifex, thet are parasite carriers), live dahnia, or well rinsed brine shrimp. if it is alive and moving it is a TREAT!