Cornstarch and tapioca flour are both silky powders known as starch thickeners. These and other starch thickeners are favored in baking because they don't noticeably alter the finished product's flavor and they don't add much fat, unlike protein thickeners. Tapioca flour is preferred for items baked at a low temperature and that benefit from the glossy sheen it imparts. While cornstarch can serve as a substitute for tapioca flour in many baked goods, it won't ever work perfectly.
Divide the required quantity of tapioca flour in your recipe by two; you only need half as much cornstarch. For example, a recipe calling for 2 tablespoons of tapioca flour requires only 1 tablespoon of cornstarch.
Measure out the resulting quantity of cornstarch with a measuring spoon or cup. Level the spoon or measuring cup with a knife for precision. Remember that baking, unlike cooking, requires exact measurements for the necessary chemical interactions to work properly.
Mix the cornstarch with an equal amount of cold water until it forms a loose paste-like consistency. Stir gently with a spoon. Add this slurry to the batter or sauce you're thickening.