Have the eggs and butter at room temperature. If you forgot to take the butter out, do not melt it, wait until the butter comes to room temperature. If you forgot to take the eggs out of the refrigerator, place them in a bowl of hot water for five minutes.

Set up a double boiler and have the water at a simmer. Cut the butter into one tablespoon portions.

Add the sugar to the top of the double boiler, then add the water and take a spatula or clean finger and gently mix them together, keeping the sugar away from the sides of the pot. Add the corn syrup. Place the thermometer in the mixture and bring to 240° F. DO NOT STIR.

In a stand mixer with a whisk attachment, mix the egg whites and cream of tartar, whip until a soft peak.

Remove the sugar mixture from the heat and very slowly and steadily start adding the sugar mixture to the egg whites with the beaters running at a slow speed. Pour the sugar mixture down the side of the mixing bowl and not directly into the egg mixture.

As soon as the sugar is all added, turn the speed up to medium high and whisk until the bottom of the bowl has cooled. This may take 10-15 minutes.

Once the meringue has completely cooled, begin adding the butter one or two tablespoons at a time. Once the butter is all added, add the shortening and any flavorings or extracts. Continue to whip for about 5 minutes or until you hear a hollow slapping.