Combine egg whites and sugar in a heatproof mixer bowl set over a pan of simmering water. Whisk until mixture registers 160 degrees, about 5 minutes.
Attach bowl to a mixer with the whisk attachment. Beat on high speed until cooled and thickened, about 10 minutes. Reduce speed to medium. Add butter, 1 tablespoon at a time. Reduce speed to low. Beat in white or semi-sweet chocolate, occasionally scraping down the sides and bottom of the bowl.
Raise speed to medium-high. Beat until well combined.
Refrigerate in an airtight container for up to 3 days. Before using, bring to room temperature, and beat on low speed until smooth.
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