Add 2 egg whites, salt, and cream of tartar to a stand mixer with whisk attachment.
Turn the mixer on low for about 20 seconds to combine the ingredients and then increase the speed to high.
After 1 minute, slowly add in 2 Tbsp + 2 tsp sugar.
Continue to beat on high speed until soft peak stage (the meringue with be slightly firm but the peak will flop when the whisk is inverted).
Turn the mixer off and set aside. Add 1/2 cup sugar and 2 Tbsp +2 tsp water to a small sauce pan and mix well to dissolve the sugar.
Add a candy thermometer to the side.
Bring to a boil over medium heat, swirling the liquid in the pan occasionally to prevent burning.
Boil until the liquid reaches 240 degrees.
This can happen quickly so don’t walk away!
Turn the mixer on low and gently add the sugar syrup SLOWLY in a stream to the egg whites.
Be VERY careful as the sugar is VERY hot.
Turn the mixer to medium speed and continue whipping until the meringue is cool to the touch (if it’s too warm, your butter will melt in the next step).
With the mixer on medium speed, add 1 Tbsp butter at a time until all of the butter is added.
Make sure each tablespoon of butter is incorporated before adding the next.
Do not be concerned when your meringue deflates some at this point.
It will gain its structure back after all the butter is added.
Turn the speed up to medium-high and whip until the buttercream is silky and fluffy.
If it remains on the soupy side, then place it in the refrigerator to chill for a few minutes, then whip once again.