In the bowl of your stand mixer, combine the eggs and salt and whisk them until the yolks and whites are combined.
Then, add in your sugar and whisk it to combine.
Next, place the bowl over a water bath of simmering water over medium heat.
Whisking occasionally, heat the eggs and sugar until the sugar is dissolved. It should feel smooth when you rub it together with your fingers.
Periodically stir the eggs and watch them to ensure they do not heat unevenly. If you don’t occasionally stir and watch the eggs, they will likely cook on the side of the bowl. Another thing to look out for is the sugar can sink to the bottom of the bowl causing uneven heating.
Store bought eggs come pre-pasteurized, so heating them to a particular temperature is unnecessary. However, you must have the sugar dissolved for the meringue to whip properly.
Then, once the eggs and sugar are ready, move the bowl to your mixer and begin to whip them at high speed using your whip attachment.
After about 5-8 minutes, the meringue will almost double in size and form into firm (but not stiff) peaks. FYI: The addition of the egg yolks will keep the meringue from becoming stiff like an all-white meringue.
Once the meringue has cooled (feel the bowl with your hand), begin to add in your butter in chunks while mixing on low.
Then, after you add in all of your butter, whip the buttercream a few more minutes until the texture changes to a whipped cream-like texture. Once you have the right consistency, add in your vanilla extract.
Stop whipping the buttercream when it does not have a strong butter flavor anymore, and it is light and creamy in texture.
While the paddle is mixing, add 2 oz of pistachio butter, and 1-2 drops of green food color to reach your desired color. Allow to mix on low to combine completely. Next, switch to your paddle attachment on your stand mixer and allow the meringue to paddle on low for 8-10 minutes. Ultimately, this will push out the air bubbles to get you the most desirable texture.
Finally, prepare a piping bag to use your buttercream, or wrap it up and refrigerate it until you are ready to use it (or store it in the freezer as you would with Swiss meringue buttercream).