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Watch the video tutorial above, review the troubleshooting tips, and read the recipe in full before beginning. Make sure all the tools you are using are completely cleaned, dried, and grease-free. A quick wipe with a little lemon juice or white vinegar is very helpful.
If you haven’t done so yet, separate the eggs first. Separate 1 egg white in a small bowl, then place the egg white in your heatproof mixing bowl. Repeat with the remaining egg whites. This way, if a yolk breaks in one of them, you don’t waste the whole batch.
Whisk sugar into the egg whites, then set the bowl over a saucepan filled with just two inches of simmering water over medium heat. Do not let the bottom of the egg whites bowl touch the water. Whisk the whites and sugar constantly until sugar is dissolved and mixture has thinned out, about 4 minutes. The mixture will be thick and tacky at first, then thin out and be frothy white on top. To test that it’s ready, you can use your finger or an instant read thermometer. Lightly and quickly dip your finger (it’s very hot, be careful) and rub the mixture between your thumb and finger. You shouldn’t feel any sugar granules. If using a thermometer, the temperature should read 160°F (71°C).
No need to let it cool down to start this next step– it’s important to begin mixing while it is still warm. Transfer mixture to the bowl of a stand mixer fitted with a whisk attachment (if you aren’t already using the metal bowl that comes with it). You can use a hand mixer instead, but this step takes awhile and your arm tires quickly. On medium-high speed, beat the mixture until stiff glossy peaks form and the meringue is no longer warm to the touch, at least 10-15 minutes. On particularly humid days, this has taken me up to 17-18 minutes. If it’s still not reaching stiff peaks, stop the mixer, place the bowl–uncovered–in the refrigerator for 10 minutes, then return to the mixer and continue beating until stiff peaks form. (This has always worked for me when it’s taking forever to reach stiff peaks.)
If the bowl and meringue still feel warm, wait until both cool to room temperature before adding the butter in the next step. Feel free to place it in the refrigerator. A warm bowl and meringue will melt the butter.
Switch the stand mixer to the paddle attachment. On medium-high speed, add the butter 1 Tablespoon at a time. Wait for the butter to fully mix in before adding the next Tablespoon. After all the butter has been added, turn the mixer down to medium speed and fully beat in the vanilla and salt, about 30 seconds.
Your Swiss meringue buttercream should be thick, creamy, and silky smooth and is ready to use on any cake, cupcake, or other confection.
If your meringue has separated, curdled, or is too thick at any point after you mix in all of the butter, place the mixture in your heat-proof bowl back over a pot of 2 inches of simmering water. Without stirring, let the edges of the meringue warm up and become liquid (the center of the meringue will still be solid), about 1-2 minutes. Remove from heat and return to the mixer. Beat meringue on low speed for 30 seconds, then switch to medium-high speed and beat until smooth, about 2 minutes. Works every time. If your mixture has become too thin and soupy after you add the butter, place the entire bowl in the refrigerator (covered or uncovered, doesn’t matter) for 20 minutes to cool down, then return it to the mixer and beat on medium-high speed until thickened. Any longer than this will solidify the butter, so only refrigerate in 20 minute spurts. If it’s still soupy, place back in the refrigerator for longer before re-whipping again. More troubleshooting tips in the post above.