• Cut the butter into 1 tablespoon pieces. The butter should be at room temperature, 65-68F. If the butter is too warm and is very soft, refrigerate it for 15 minutes to firm it back up. If the butter is too warm when it is added to the meringue the buttercream won’t set and will remain too soft.
• Half fill a 3-quart or 4-quart saucepan with water and bring it to a simmer over high heat. Once it starts to simmer, reduce the heat to medium.
• Place the egg whites, sugar, and kosher salt in the bowl of a stand mixer and set it over the pan of simmering water. Do not allow the bottom of the bowl to touch the water.
• Whisk until the mixture by hand until reaches 170°F. Once the meringue mixture reaches temperature, immediately put the bowl on the mixer stand, attach the whisk attachment and whip at medium-high speed until the meringue mixture is cool (about 80-85°F).
• With the mixer on medium speed add the butter to the meringue 1-2 tablespoons at a time.
• Initially, the volume of the meringue will decrease dramatically; it may even seem soupy along the way, but as the cool butter is added, the mixture will begin to thicken. Continue to add the butter and eventually the buttercream will begin to look creamy and smooth.
• Scrape down the sides and bottom of the bowl. Add the vanilla extract and mix on medium speed until well incorporated.

Storage

• The buttercream can be made up to one day before ready to use. If making the buttercream in advance, it can be stored in the refrigerator for up to 2 weeks in an airtight container and the freezer for up to 3 months in an airtight freezer-safe container.