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1
Puree strawberries in a food processor.
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2
Combine egg whites and sugar in the heatproof bowl of a standing electric mixer set over a pan of simmering water.
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3
Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips).
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4
Attach the bowl to the mixer fitted with the whisk attachment.
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5
Starting on low and gradually increasing to medium-high speed, mix until stiff (but not dry) peaks form.
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6
Continue mixing until the mixture is fluffy and glossy, and completely cool (test by touching the bottom of the bowl), about 10 minutes.
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7
With mixer on medium-low speed, add the butter a few tablespoons at a time, mixing well after each addition.
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8
Once all butter has been added, scrape down sides of bowl with a flexible spatula and switch to the paddle attachment; continue beating on low speed until all air bubbles are eliminated, about 2 minutes.
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9
Add strawberries and beat until combined.
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10
Stir with a flexible spatula until the frosting is smooth.
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11
Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month.
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12
Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes.
