Combine sugar and egg whites (at room temperature) in top of a double boiler; add water, and beat on low speed of electric mixer for 30 seconds or just until blended.
Place over boiling water; beat constantly on high speed of electric mixer about 7 minutes or until stiff peaks form. Remove from heat. Add rum, and beat 2 additional minutes or until frosting is thick enough to spread.
Oxmoor House Homestyle Recipes