Have all ingredients pre-measured. Have on-hand a spatula, candy thermometer, pastry brush, metal spoon, and cup of water.
Cut butter into small pieces and lay out on a piece of parchment paper to help it get to room temperature.
In a saucepan, heat water and sugar. You will want to bring the sugar up to hard candy temperature. While heating, scrape any residue from the top and sides of pan with metal spoon and wet down the sides of the pan with wet pastry brush. Once you have done this, leave the pan alone.
Once the sugar gets to soft candy temperature, begin whipping the egg whites in kitchen-aid mixer with whisk attachment. Once the sugar is up to hard candy temperature and the egg whites are frothy, slowly pour the sugar into egg whites while continuing to whisk.
Keep the mixer whisking until mixture is cooled to room temperature. You will be able to test this by touching the side of the bowl. Once the bowl feels the same temperature as your hand, it is ready.
Once cooled to room temperature, add butter quickly, add vanilla and whip on med-high until the mixture becomes light and fluffy and holds a peak.