Before starting on this buttercream, it's crucial that your mixing bowl is super clean; otherwise, the egg whites will fall flat. Make sure to wash your mixing bowl with degreasing soap and white vinegar to rid of any residual grease.
Place the egg whites into the stand mixer bowl and whisk on high speed for 2 to 3 minutes, just until stiff peaks form; set aside. Into a separate sauce pan, add the sugar and water but do not stir! Cook the sugar over medium heat until it reaches soft ball stage or 238F, about 7 minutes. With the mixer running on medium speed, immediately pour the hot sugar syrup in a slow and steady stream into the egg whites.
Once all the syrup has been added, turn the mixer from medium speed to high and continue whisking for 6 to 7 minutes. The egg whites will be super fluffy, firm and will be slightly shiny. Test the temperature of the bowl before adding the butter. Place your wrist against the bottom of the bowl; when it's just lukewarm, begin adding the butter.
Add the softened butter 1 to 2 tablespoons at a time, with 10 to 15 seconds between additions. Scrape down the sides of the mixing bowl often to get everything incorporated. Once all the butter has been added, the buttercream will become more smooth. Turn the mixer to high speed and whisk for another 4 to 5 minutes. Add any extracts, chocolate or flavorings at this time.
Buttercream can be stored in the refrigerator if not used right away. Thaw at room temperature, then whisk again before using.