This fluffy buttercream is lightened with an Italian Meringue made by whipping sugar syrup into egg whites. The meringue strengthens the buttercream, creating a stability that allows for building extra-tall layer cakes the Milkmoon Kitchen way!
Place the sugar and water in a medium saucepan and set over medium heat, stirring occasionally. Use a pastry brush dipped in water to melt the sugar crystals that form on the sides of the pan. When the syrup begins to boil, reduce the heat to low and insert a candy thermometer into the syrup. Allow it to continue to boil while moving on to the next step.
Place the egg whites in the clean bowl of a stand mixer fitted with a whisk attachment. Whip the whites until frothy. Then, if using, sprinkle in the pinch of cream of tartar (this improves the strength of the meringue).
Keeping an eye on the temperature of the sugar syrup, increase the speed of the mixer to high to achieve stiff peaks. Ideally, you will time this to coincide with the sugar syrup reaching 248°-250° F, the Firm Ball Stage. If your whites reach stiff peaks before the sugar syrup reaches the correct temperature, reduce the mixer speed to the lowest setting and leave running until the syrup is ready.
When the syrup reaches 248°-250° F, increase the mixer speed to the highest setting. Remove the saucepan from the heat and slowly pour the hot syrup down the side of the bowl into the egg whites, being very careful to avoid allowing the stream to hit the whisk and spatter. Pour slowly and steadily until all the syrup has been added, and continue beating the meringue until it becomes light and fluffy and holds its shape exactly when the beater is removed, roughly 3-5 minutes.
After this time, the meringue may still be hot—if so, place the bowl in the refrigerator (or freezer) for 10-15 minutes until it has cooled all the way through.
When the meringue has cooled, place the bowl back on the mixer, replacing the whisk with the paddle attachment. Begin mixing on low, and add the butter 1/4 cup (half a stick) at a time, beating well after each addition. Once the butter is added, increase the speed to medium and beat the buttercream until it becomes light and fluffy. It may look soupy or curdled shortly after all the butter is added—this is okay! Just keep on beating it and it will whip up into a light, fluffy cloud.
Add the pinch of salt and vanilla, if including, and beat on low speed until incorporated. You may now add any other flavorings, gel or liquid food colorings, shaved chocolate, chopped nuts, fruit purées, or any other goodies you like to add to your buttercream. This is a very versatile recipe, so play around and create your own delicious flavors!