Make-Ahead, Storing, & Freezing Instructions: Swiss meringue buttercream is great left covered at room temperature for 1-2 days, but after that, refrigerate it for up to 5 days or freeze up to 3 months. If freezing, store in an airtight container, then thaw it at room temperature on the counter. Once completely at room temperature, about 72°F (22°C), place into the bowl of your stand mixer fitted with a paddle attachment and beat for 2-3 minutes until creamy again.
Yield: This recipe yields about 5 cups of frosting, which is enough to fill and frost a two layer 9-inch cake with a generous amount, to fill and frost a three layer 9-inch cake with just enough frosting, to lightly frost 2 dozen cupcakes, to heavily frost 1 dozen cupcakes, or a 9×13 inch sheet cake (with some frosting leftover).
Egg Whites: For best success, I recommend using fresh eggs instead of carton egg whites. Here are all my recipes using leftover egg yolks. Eggs separate much easier when they’re cold. Separate the egg whites one at a time and place the egg white into a large heat-proof mixing bowl (or a double boiler or the metal mixing bowl from your stand mixer) before separating the next. This way, if a yolk breaks in one of them, you don’t waste the whole batch.
There’s almost always a way to fix “ruined” Swiss Meringue Buttercream and it usually has to do with temperature. See step 8 as well as my troubleshooting tips in the post above.
Butter: This buttercream will thin out and become liquid-y if the butter is too warm. Make sure you’re using butter that is slightly cooler than proper room temperature butter. Remove butter from the refrigerator and set it out for just 30-40 minutes before you need to add it to the meringue. Sometimes meringue takes longer than usual or it needs a break and in that case, you should place the butter back into the refrigerator for a few minutes so it doesn’t get too warm sitting out. The butter should be cool to the touch. For accuracy, I recommend using an instant read thermometer. Butter should be 60°F (16°C).
Flavors & Colors: For coloring, it’s best to tint this frosting with gel food coloring so you aren’t adding a lot of extra liquid. Beat food coloring into the frosting on low speed after you add the vanilla extract and salt.
Half or Larger Batch: You can halve this recipe. The egg white/sugar mixture won’t take as long to cook and the meringue won’t take as long to reach stiff peaks. I do not recommend making larger batches at a time because it will quickly overcrowd and wear out your mixer. If you need more batches, make them all separately.