Food stamps

We are having a small dinner for my fiancé daughter and money is tight and it will be just close family members. My fiancé is away at training and his grandmother had offer to pay for the cake for her grand baby using food stamps. I told her that it was no need for that and I will order a cake myself. Shes very old, doesn't take no for a answer and I understand her love for helping (I would never look down upon someone's for public assistance) she had already order a cake. i don't know anything about food stamps, my questions: Is it possible to purchase a custom cake with foodstamps? I mean fully custom not one were you just pick a cake from the fridge and put name on it. Should I have a back up cake?