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1
In a heat proof mixing bowl, combine egg whites, sugar, and pinch of salt. Place over a pan with about 2 inches of simmering water. The bowl should NOT touch the water.
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2
Whisk constantly in the double-boiler for about 3 minutes, until all the sugar granules are dissolved. To test, rub some of the mixture between your fingertips; it should be smooth and slippery with no grains remaining.
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3
Remove from heat and place the bowl on your stand mixer or a hot mat if using a hand mixer (and yes, this CAN be done with a decent hand mixer). Whip meringue mixture on medium speed for 5 to 10 minutes, or until cooled and stiff peaks have formed.
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4
Add the very soft butter one stick at at time, incorporating completely before adding the next. DO NOT PANIC if your mixture curdles or appears soupy after adding the butter. Continue whipping on high speed until it comes back together. This can take up to 15 or 20 minutes for some people, but it usually only takes about 10 minutes. Some folks don't have trouble with this step at all. The frosting should whip up very soft and smooth but still able to hold firm peaks.
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5
Add flavoring extract of your choice at this point. Basic vanilla is lovely; lemon extract is also delicious. If coloring, paste food color is preferred over liquid drops to avoid adding excess moisture to the buttercream.
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6
This buttercream may be used immediately and is suitable for both spreading and piping. It can also be stored in an airtight container in the refrigerator for 3 or 4 days (with plastic wrap touching the surface of the frosting) or packed in the freezer for up to 1 month. Thaw in the fridge and allow refrigerated buttercream to sit out on the counter until it comes to room temperature before whipping on medium speed and spreading/piping.
