Let's make the best Swiss Meringue Buttercream!

1. Separate the eggs: 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.
2. Whisk sugar & vanilla extract into the egg whites.
3. Set the bowl over a saucepan filled with just two inches of simmering water over medium heat, making sure not to let the bottom of the egg whites bowl touch the water. Whisk constantly until the 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. You shouldn’t feel any sugar granules. If using a thermometer, the temperature should read 71°C.
4. Move the bowl straight to your stand mixer with whisk attachment. No need to let it cool down to start this next step—it’s important to begin mixing while it is still warm. 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 8-15 minutes. If it is a particularly humid summer day, it can take up to 17 minutes. If you are struggling to get it to reach stiff peaks, place the whole bowl (with whisk) into the fridge for 10 minutes to allow it to cool slightly. Then return to the mixer and continue beating until stiff peaks form. 
5. If the bowl and meringue still feel warm, wait until both cool to room temperature (around 21°C) before adding the butter in the next step, as a warm bowl and meringue will melt the butter. You can place it into the fridge.
6. 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. 
7. Your Swiss meringue buttercream should be thick, creamy, and silky smooth and is ready to use on any cake, muffin or baked good!

If Too Thick or Too Thin: 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.

Recipe from Sally's Baking Addiction