From the Super Mario Wiki
A Muku Cookie is a special cookie found in Super Mario RPG: Legend of the Seven Stars. Mario can buy one in Seaside Town for 69 Coins (only before beating the boss Yaridovich though; in fact, Mario can only buy the cookie from Yaridovich himself). When used, the party summons a Mukumuku, who tries to hit a member of the party, but fails. The creature then destroys himself by throwing a bomb. For some reason, this recovers 69 HP for all party members. Muku Cookies can also be acquired by using a Yoshi Cookie on a Mukumuku in battle, as Yoshi will arrive and transform the gopher into the unique cookie.