Combine the sugar with water in a small clean pot over very low heat. Make sure that the sugar has all dissolved in water without stirring it. Instead, swirl the water inside the pot.
Once all the sugar melts, raise to medium heat and use the thermometer resting in the pan to read the temperature, which should be about 230 to 240 degrees.
To prevent sugar crystals from forming up around the sides, wash them down with a wet pastry brush. Leaving crystals in the sides is enough to make the meringue unstable.
Meanwhile use a hand or bowl whisk to whip the egg whites on low speed until they’re foamy.
Slowly add the cream of tartar, and gradually raise the speed to about medium. Keep going until the meringue forms those soft sparrow peaks as you put the whisking attachment out of the mixture.
Continue beating the eggs on medium speed and very slowly start pouring the hot sugar syrup over the egg whites. Make sure to do that in a very thin stream and continue beating until the egg whites become glossy, stiff, and ready for use.