Baked goods usually require a starch for binding, rising and thickening. Recipes call for a particular starch based on its unique characteristics when cooked. Tapioca flour is a root starch, which means that it cooks faster, gives bread and cookies a chewy texture, and adds a glossy sheen to the finished recipe. Cornstarch is a grain starch. It takes longer to cook and doesn't impart the same glossy luster of tapioca flour. Keeping these differences in mind, it is possible to substitute cornstarch for tapioca flour in baking with good results.
Determine the amount of cornstarch required for your recipe. When using cornstarch in a recipe that calls for tapioca flour, the substitution is 1 to 2, or half the amount of tapioca flour. For example, if the recipe calls for 2 tablespoons tapioca flour, use just 1 tablespoon cornstarch.
Measure the cornstarch and mix into the recipe as directed in place of the tapioca flour. When baking breads and cream based pies, cornstarch works as well as tapioca flour. For fruit pies the glossy appearance created by the cooked tapioca flour is desirable but not crucial.
Bake the mixture as indicated by the recipe. Cornstarch takes longer to cook than tapioca flour so it may need to bake slightly longer in order to yield the same effect.