Meringue powder is simply dried egg whites with stabilisers and anti-clumping agents such as cornstarch. You will likely not need to add additional starch if you are careful to keep this meringue powder substitute in a cool, dry place. It is often required in frosting and other dessert recipes to add lift and stiffness. It can be mixed with water and superfine sugar. The result is a foam that is good for using in angel food cake or as a topping for a meringue pie. Since it is made from dried egg whites, meringue powder does not have the same risk of salmonella that raw eggs have, but this powder can be difficult to find. If your recipe requires meringue powder, here is how to make your own for use in recipes.
Look at the amount of meringue powder and water needed in your recipe.
Determine the number of egg whites you will need to replace by the number of tablespoons of meringue powder required. Each tablespoon of meringue powder mixed with 2 tbsp of water equals the white from one egg.
Replace each tablespoon of meringue powder with 2 tbsp of powdered egg whites and keep the water at the same amount. Two tbsp powdered egg whites is the same as the white from one egg.
Continue with your recipe as directed. Store the powdered egg whites in a cool, dry place, tightly sealed.
Dried (powdered) egg whites are in the baking aisle at most grocery stores.