Prepare your ingredients and workspace.

Combine sugar and water in a small saucepan. Place egg whites in a stand mixer bowl. Set up your workstation to monitor both the sugar syrup and the meringue.

Simultaneously heat the sugar syrup and start whisking the meringue.

For traditional Italian Meringue/raw egg whites: Start whisking egg whites on low. Heat the sugar syrup on medium-high. When the syrup reaches 220°F (104°C), increase the mixer speed to medium-high until soft peaks form. Continue heating syrup to 245-248°F (118-120°C). If your meringue reaches soft peaks before the syrup is ready, turn the mixer to low speed.

For pasteurized (carton) egg whites: Whisk the egg whites at medium speed. Heat the sugar syrup to 245-248°F (118-120°C). The meringue might not foam yet, which is fine; immediately proceed to Step 3.

Pour the sugar syrup into the whipping meringue. Check to make sure your stand mixer is moving at medium speed. Once your syrup has reached 245-248°F (118-120°C), immediately and slowly pour the hot syrup into the egg whites. You want to aim for where the edge of the mixing meringue meets the bowl.

Mix until you get a soft, runny foam (this will take a few minutes with pasteurized egg whites) or stiff peaks (up to 5-10 minutes or more for a traditional Italian meringue). Pour the slightly warm meringue into a bowl.

Make the Frosting Butter. In the bowl of a stand mixer, beat slightly softened butter on high speed for 2 minutes until light, smooth, and creamy. Scrape down the sides as needed.

Add the powdered sugar, vanilla extract, and a small pinch of salt to the butter and whisk again at medium-high speed for another 1-2 minutes.

Add the meringue to the frosting butter. Ensure that your meringue is at room temperature and add it to the Frosting Butter in 2 to 3 additions, whipping for about 1 minute after every addition. Once all the meringue has been added, mix for an additional minute at high speed.

Switch to the paddle attachment and mix on low speed for a few minutes to smooth. Taste and adjust salt and vanilla as needed.