Italian Buttercream

Ingredients

• 5 large egg whites (about 150 grams)
• 0.125 teaspoon citric acid
• 1.5 cups granulated sugar
• 0.5 cup water
• 1 pound butter, cut into 1-inch cubes, softened
• 1 teaspoon salt
• 1 teaspoon vanilla extract (or vanilla bean paste) or 2 teaspoons lemon zest, for flavoring (optional)

Steps

1. Put the egg whites and citric acid in a stand-mixer bowl fitted with the whisk attachment. Run on low while preparing the sugar syrup.
2. In a medium saucepan, stir sugar and water over medium heat until dissolved, about 2 minutes. Raise to medium-high and bring to a boil. Boil without stirring; if crystals begin forming, brush down the pot's sides. When an instant-read thermometer reads 239°F, remove from the heat. Set the stand mixer to medium and immediately stream all the syrup very slowly into the whites along the bowl's inside edge. Increase to high and beat about 5 minutes, until the meringue is completely cool.
3. With the mixer on low, add butter to the meringue 1 piece at a time, increasing the speed to high. If it does not come together at first or deflates slightly, keep mixing on high until it comes together. Scrape the sides and bottom of the bowl, then beat in salt. Add vanilla extract or lemon zest if desired. Refrigerate in an airtight container for up to 3 weeks or freeze for up to 3 months. Defrost completely at room temperature, then whisk again in the stand mixer to bring the buttercream back together before use.