Combine the sugar and egg whites in your stainless steel mixer bowl (or any heat proof bowl).

Attached your candy thermometer to the side of the bowl.Set your bowl over top of a pot with water, just enough to cover the bottom of the pot. You do not want the water to boil and touch the bottom of your mixing bowl.

Heat the water on medium to medium-low heat.

Combine the sugar and egg whites and stir (using your greaseproof spatula) constantly until the mixture reaches 140° F. This is the most important part; you do not want to egg whites to cook. I have read that you can have it be anywhere between 120° F – 160° F. When it reaches 140° F, the sugar should be dissolved in the egg whites. You can test this by placing a drop on your fingertips and rubbing them together. The mixture should be completely smooth.

Place the mixing bowl on your stand mixer. Using the wire whisk attachment, beat the egg whites. Start on low and increase the speed to medium after 2 minutes. Beat for another 3 minutes on medium. At this point, your frosting should have turned white as the egg whites are whipped and the body of the meringue starts to form. After 5 minutes, increase the speed to high. You will notice that as you increase the speed, the body of your meringue will also increase. It will start to look glossy as the peaks begin to form. Beat for another 5 minutes. After you have beat the meringue for a total of about 10 minutes, the bottom of your mixing bowl should be cooled completely. Your SMB should look kind of like whipped cream.

Scrape down all sides and bottom of the bowl.Turn your mixer down to medium-low. Slowly add your butter, about 1 tablespoon at a time, ensuring it is mixed well after each addition. Once you have added all your butter. Mix for another minute and scrape down the sides of the bowl.

Switch to the paddle attachment and continue beating on medium to allow all the air bubbles to escape. In about 2-3 minutes the frosting is smooth. Lastly, fold in the lemon zest.