Not my betta, but my molly does this. I used a reed to tuck the wafer under a rock that she can't get to easily. Otherwise she pigs out on it, and not even mentioning how aggressive it makes her. She's like "This food is MINE!" and goes all Hulkamania on anyone who swims near.
One time though, I didn't tuck it far enough and she dug a wide enough rut out in the gravel and got to it. I was getting ready for school after I fed them. I walked by it on the way to the shower and didn't see her and thought, hmm, I guess she is hiding in the ship. Took my shower, came out, and out of the corner of my eye, I see a white flash in the back (she's silver). Yup, she'd dug a hole to it, and she was trying to eat it all sneakily. She'd nibble a bit, raise up over the rock, check if the coast was clear, and then go back for another nibble. She was FAT!