No. Baking powder is a leavening agent which helps baked goods to rise while cornstarch would be considered a thickening agent. In a pinch, you can combine the following ingredients to make 1 teaspoon of baking powder (equals 4 grams) Homemade Baking Powder Substitute - 1/2 teaspoon cream of tartar - 1/4 teaspoon cornstarch - 1/4 teaspoon baking soda