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Swiss Meringue Buttercream 101

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Ingredients

Lyndsay's Swiss Meringue Buttercream Frosting

  • 1 cup (8 ounces) large egg whites, from about 7 to 8 large eggs
  • 2 1/4 cups (15.75 ounces) granulated sugar
  • 2 1/2 cups (20 ounces) unsalted butter, cut into 1-inch cubes and at room temperature
  • 2 teaspoons pure vanilla extract
  • a pinch of kosher salt

Instructions

  • In the bowl of a freestanding electric mixer, combine 1 cup large egg whites and 2 1/4 cups granulated sugar. Mix with a heatproof rubber spatula to combine into a sugary slurry. Fill a medium saucepan with a few inches of water and place it on the stovetop over medium-high heat. Set the mixer bowl on top of the saucepan to create a kind of double boiler, making sure the mixer bowl doesn't touch the water.
  • Heat the egg and sugar mixture until it reaches 160 (F) on the candy thermometer, using the rubber spatula to scrape down the sides of the bowl to prevent any ingredients from burning and stirring the mixture occasionally. This process should take around 10 minutes, so if you find that it's taking longer, don't be afraid to crank up the heat!
  • Use oven mitts to carefully transfer the mixer bowl onto the stand mixer and fit a whisk attachment in place. Turn the mixer on to its highest setting and beat the mixture for 10 minutes, or until you've created a meringue with medium-stiff peaks and the sides of the bowl are only slightly warm or at room temperature.
  • Scrape down the whisk, and replace it with the paddle attachment. With the mixer on low speed, add 2 1/4 cups cubed unsalted butter a few cubes at a time, continuing to mix until the butter is incorporated. At this point, it should look like a thick soup, and that's totally okay. Add 2 teaspoons pure vanilla extract and a pinch of kosher salt, and turn the mixer up to its highest speed. Beat the mixture on high speed for about 2 to 3 minutes, or until a fluffy, silky, magical buttercream frosting has formed.

Notes

 
Troubleshooting Swiss Meringue Buttercream Frosting

... if your Swiss Meringue Buttercream is too soupy:
That likely means that the meringue wasn't cool enough when you started adding the butter, and/or it's a super hot day, and/or your butter was way too soft. No worries! Place the whole bowl in the fridge for about 10 minutes to cool it down before beating it again, first on low speed and gradually increasing it to high speed.
... if your Swiss Meringue Buttercream looks curdled and/or mottled:
That likely means that the butter you used was way too cold and refuses to emulsify with the meringue. We can fix it, don't even worry. Eyeball out a 1/4 cup scoop of the curdled/mottled Swiss Meringue buttercream and transfer to a liquid measuring cup. Microwave on low speed for about 15 to 20 seconds, or until the frosting is soupy, liquidy, and warm. Pour back into the rest of the frosting and turn the mixer back on to high speed for about 2 to 3 minutes — the melted frosting should have helped bring the entire batch down to the right temperature for whipping.
If you don't have a microwave, you can try Lyndsay's suggestion: lightly heat the whole bowl over a low-boing pot of water. Once it's warmed up a teensy bit, try mixing it again.

Storing Swiss Meringue Buttercream Frosting
Swiss meringue buttercream will keep in an airtight container for up to 1 week in the fridge or up to 1 month in the freezer. Just be sure to store in an airtight container to prevent the frosting from absorbing any weird fridge/freezer flavors.
To use chilled buttercream, bring it back to room temperature by letting it sit uncovered at room temperature for about 1 hour or so. Transfer to the bowl of a freestanding electric mixer and gently rewhip before using. If you find that the buttercream looks mottled, use the tricks above to incorporate it fully. To use frozen buttercream, thaw the buttercream in the fridge overnight and follow the steps for thawing chilled buttercream.