Tenzo came to the decision to involve the restaurant Kakashi frequents most, Ichiraku Ramen. Perhaps if they started offering the cookies, then Kakashi would more naturally try them. All he had to do was convince the chef, Teuchi, to follow his plan.
Tenzo arrived at the Ramen stand, thankfully empty. He approaches the chef, who's busy preparing ingredients for the no doubt busy day ahead.
"Hello Teuchi, my name is Tenzo and I'm concerned about a dear friend of mine that you know, Kakashi Hatake. Recently he's been skipping meals, mostly due to forgetting to eat them, and I was hoping you could help me get his calories back up so he can go back to a healthy amount. I've designed these cookies to have enough calories to compensate for half of a normal meal per cookie, about 500 calories. Could you please offer these to him, maybe he won't be so stubborn with you."
Teuchi agrees immediately, although without all the information. The cookies contain about 2,000 calories each, and Kakashi never skips meals, as his diet is highly regulated. But this is all to help Kakashi in the end, so Tenzo doesn't feel bad twisting the truth.
Teuchi implements the cookies that day, as Naruto drags Kakashi to the stand regularly. Surprisingly Kakashi is willing to try them, immediately finding a liking to them, and having three cookies. Naruto, jealous of his sensei getting a special treat, demands to have some aswell. Teuchi gives in, a soft spot for Naruto outwieghing the agreement he made, and Naruto ends up eating 5 of them. Kakashi and Naruto leave the stand significantly more bloated than usual, but just dismiss it.
Because the two of them frequent the stand so much, they usually will have at least three of the cookies per day. And with them going to the stand for ____ weeks after the cookies began. The effects are plainly obvious.