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Melt your white chocolate and cream in a small pot on low heat over the stove OR in the microwave in 20 second increments, stirring often. Bring it to ROOM TEMPERATURE before you try to add it to the buttercream, or else you'll have a mess. Make sure that you give it a stir now and then so it doesn't solidify.
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Fill a small to medium-sized pot with 1 inch of water, and bring to a boil on the stove.
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In a metal mixing bowl, add your egg whites and sugar, and place over the pot. Make sure that no part of the bowl is touching the boiling water, but rather sitting over it.
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Whisking constantly, bring the eggs and sugar to 140°-160°F. If you stick your fingers in, you shouldn't feel any sugar granules, but the mixture is still warm, not hot. It will look quite foamy.
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Using the whisk attachment for your mixer, set the speed to high, and whisk your meringue until it's glossy, smooth, and forms stiff peaks. (About 3-7 minutes)
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Set the speed to low, and add in your cubes of butter, waiting a few seconds in between each one. Let it mix. It'll go through some pretty gross-looking stages, but just be patient, eventually you will end up with a beautifully smooth, light buttercream! (About 5-10 minutes)
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Keeping the speed on low, pour in your room temperature white chocolate and wait until it's all mixed in.