Making Swiss Meringue Buttercream

• Fill a large pot with about 2 inches of water and bring it to a simmer.
• Place the metal bowl of your stand mixer on top of the pot of water to create a bain-marie (or double boiler). The water should not touch the bottom of the bowl.
• Whisk together the egg whites, sugar, and salt in the mixing bowl. Pro-tip: Wipe your bowl out with white vinegar before making the meringue. A very clean metal bowl will work best for this as the meringue is easier to whip up in metal rather than plastic.
• Cook the egg white mixture until it reaches 110ºF (43ºC) and constantly whisk the sides of the bowl to distribute the heat evenly. If you don't have an instant-read thermometer, when you can no longer feel any sugar granules in the egg whites with your finger, it's done.
• Remove the bowl from the heat and transfer it to your stand mixer with the whisk attachment. You can use a hand mixer and a large bowl to mix your meringue, it will just take a lot longer to mix.
• Whip the egg and sugar mixture on medium-high speed for 10 to 15 minutes, or until you reach glossy, stiff peaks. 
• Pour your meringue out into a shallow dish and refrigerate it for about 10 minutes to cool the meringue to room temperature. You can also put your entire bowl of meringue into the fridge, but it will take longer to cool. If you don’t cool the meringue it will melt your butter and then you’ll have buttercream soup.
• Once your meringue is cooled, put it back in your stand mixer bowl with the whisk attachment. 
• Add the room-temperature butter, vanilla, and mix until it’s all combined. 
• Whip the buttercream on medium/high speed until it is white, fluffy, and doesn't taste buttery, this can take 8-10 minutes. Pro-Tip: To counteract the yellow in your buttercream, add a drop or two of violet food color gel to make it whiter.
• Switch to a paddle attachment and mix it on low speed for 15-20 minutes to make the buttercream very smooth and remove air bubbles. This isn't required, but if you want really creamy frosting, you do not want to skip it!
• This recipe is enough to frost and fill a two-layer 8" x 2" cake, or frost about 24 cupcakes. Store any leftovers in an airtight container in the fridge for 1 week, or freeze for up to 6 months.