What is the "に" for?
It changes a noun ほんとう truth into an adverb ほんとうに truly.
Can someone help me figure out why ほんとうに here instead of とても?
I think it is the difference between it is really delicious versus it is very delicious.
Why hontoni instead of taihem or totemo?
I think, in normal speech, any of these three would communicate that "it's better than just good," so using ほんとうに here may just be for the sake of teaching that it is another way to say that.
ほんとうに on it's own is like asking "really?" So ほんとうにおいしい is more like saying "really delicious" or "truly delicious," while とてもおいしい would mean "very delicious."