Measure out all of your ingredients and set out all of the equipment needed.
Cut the butter into 1-2 tablespoon chunks. It should be very soft but not melted.
Place the egg whites in the mixing bowl of the stand mixer and let them come to room temperature.
Whip the egg whites and slowly add in 1/4 cup of white sugar. Continue beating until they are a meringue at a soft-peak stage.
Place the candy thermometer on the edge of the small pan, adjusting it so that the temperature reader isn’t touching the bottom or edge of the pan.
Combine the sugar and water in it and cook over medium-high heat, occasionally stirring to help the sugar dissolve.
Cook the sugar syrup until it reaches 235°-240°. This is also called the softball stage.
While the mixer is running on medium speed, slowly drizzle the hot sugar syrup into the meringue in a steady stream.
Continue whipping the egg whites until they reach a stiff peak stage.
While the mixer runs, add the vanilla extract and the butter one piece at a time. Allow it to beat for a few seconds between adding each piece. The buttercream will look soupy and “broken” right after some of the butter has been added. However, if you keep beating it while adding the rest of the butter, it will come together and become a light and fluffy frosting.