In most mochi recipes, tapioca starch is not a good substitute for glutinous rice flour, and it tends to yield a rubbery, tough texture that is noticeably different from the soft and supple chewiness that comes from glutinous rice flour. Fortunately in this recipe, the fruit puree helps to keep the waffles tender and the substitution isn’t as noticeable when glutinous rice flour is replaced with tapioca flour. To substitute, replace glutinous rice flour for tapioca flour with a 1:1 ratio. You can also use potato starch and arrowroot starch. On the other hand, cornstarch is NOT recommended to be used in this recipe.