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1. Over a large pot of gently simmering water, create a double boiler with stand-mixer bowl. Make sure the pot that you use is just big enough so the bowl fits inside without touching the water. Whisk egg whites and 1 cup plus 2 tablespoons sugar in that mixing bowl. Continually whisk the mixture over the simmering water until sugar dissolves and you are unable to feel any sugar granules when rubbed between fingertips, about 5 minutes. Be careful not to cook the egg.
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2. Remove bowl from heat and put it onto your stand mixer. Beat egg whites (with whisk attachment) until egg whites form stiff peaks and are completely cool (the bowl should also be cool), about 15 minutes.
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3. One square at a time, add butter while mixer is continually whisking the meringue. Continue to do this until all butter is incorporated. You want your buttercream to be light and fluffy here. If it's not, and looks broken (caused if the butter is too warm) all you need to do is continue whipping with cold packs or ice under your bowl. As soon as that butter starts to firm up from the cold, your buttercream will whip back into shape. Literally. Add vanilla extract and seeds from the 1/4 of a vanilla bean pod, if using.
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4. Drain off any oil that is on top of the room temperature natural peanut butter and then stir really well before using. Gradually mix in 2 cups of the peanut butter, making sure to scrape down sides of the bowl as you go. Refrigerate 20-25 minutes before using to set up.
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5. Re-whip and use!
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*If using the frosting the same day, keep it room temperature with plastic wrap on top. If using the next day or later, refrigerate (up to 3 days). Can also freeze up to a month. For refrigerated buttercream, bring back to room temperature on the counter before using then beat with a paddle attachment. For frozen, defrost overnight in the fridge then follow refrigerated instructions.
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Recipe makes approximately 5 cups.
