It happens. You open your pantry and the cornstarch is gone. Maybe a ghost used it for pudding? The horror.
No worries! You can use arrowroot powder or potato starch in the exact same way. Same ratio: 1 cup AP flour minus 2 tablespoons, plus 2 tablespoons of the starch.
Pro tip: Don’t use tapioca starch. It gets a little gummy. We’re making cake, not boba tea (though boba tea is lovely).