Regular all-purpose flour contains gluten, which gives baked goods their structure. But, if you add too much flour or overwork the dough, it could get dense! Cornstarch, on the other hand, is gluten-free. When cornstarch is mixed with flour, it helps create a lighter, more tender shortbread cookie. Note: Cornstarch is added to store-bought icing (powdered) sugar as an anti-caking agent. So, shortbread recipes that call for icing sugar also benefit from the trace amounts of cornstarch added during processing. But, when using icing sugar, it is hard to know exactly how much cornstarch is being added. For the most consistent results—and for a higher ratio of cornstarch—we prefer adding it separately, like in this Earl Grey Shortbread recipe.