Place room temperature egg whites in the mixer bowl, but do not whip. Just get ready and set aside.
Place a cup of sugar and ¾ cup of water in a saucepan. Insert a candy thermometer and heat over medium heat.
Once the sugar syrup reaches 230°F, start whipping the egg whites adding a bit of the remaining sugar at a time until the sugar is all added and the egg whites start to form stiff peaks.
As soon as the sugar syrup reaches 240°F, remove it from the heat and immediately start drizzling it into the whipped egg whites with the mixer on high. You want a slow and steady stream and you need to be really careful, that syrup is HOT!
Keep beating on high speed until the outside of the bowl is no longer warm, this can take about 15 minutes.
Switch to the paddle attachment and turn the mixer back on to medium speed. Add the butter, about a Tablespoon at a time. Allow each addition to mix in completely before adding the next one.
It may look funky a couple of times along the way, just keep the faith and keep mixing. Once all of the butter is added, continue to beat on medium speed for 5-10 more minutes or until super creamy and smooth.
Add a pinch of salt and your desired flavorings and mix well.