Buttery Chocolate Frosting
Note: Begin 20 minutes ahead
1. In heavy, 1-quart saucepan over medium heat, heat whites, sugar and rum extract until sugar is dissolved, about 2 minutes, stirring constantly (over-heating will cause egg whites to coagulate).
2. In large bowl with mixer at medium speed, beat butter until light and fluffy, about 4 minutes (if butter becomes too warm, frosting will separate).
3. Gradually beat egg-white mixture into butter; beat in cocoa. At high speed, beat until the mixture is light and fluffy, about 2 minutes.