1. In a medium heatproof bowl set over simmering water, combine sugar and egg whites; stir until hot and sugar is dissolved, about 5 minutes. Transfer to a mixer bowl and beat to firm peaks, about 8 minutes. While mixer is running, add butter, a few tbsp. at a time, until completely combined. Reduce speed to low; beat in chocolate and extract. Use immediately. (Can be made ahead. Store in an airtight container in the refrigerator up to 2 weeks. Bring to room temperature, then beat on low speed until smooth before using.) Makes 10 cups.