Put sugar and egg whites in the heatproof bowl of an electric mixer, and set over a pan of simmering water. Whisk constantly until sugar is dissolved and mixture registers 140 degrees on an instant-read thermometer.
Transfer bowl to an electric mixer fitted with the whisk attachment; beat on medium-high speed until fluffy and cooled, about 10 minutes. Continue beating until stiff peaks form.
Reduce speed to medium-low; add butter by the tablespoon, beating well after each addition. Beat in vanilla and white chocolate. If not using immediately, refrigerate in an airtight container up to 3 days or freeze up to 1 month. Bring to room temperature before using; beat on low speed until smooth, about 3 minutes.
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