Two kinds of chocolate -- cocoa powder and the finest semisweet chocolate -- make this rich, dark mousse.
In medium bowl, stir cocoa powder with 6 tablespoons warm water; stir in rum. In another medium heatproof bowl or top of double boiler set over a pan of barely simmering water, heat chocolate, stirring occasionally, until melted. Stir melted chocolate into cocoa-powder mixture. Set aside; keep warm.
In medium heatproof bowl or clean top of double boiler, combine sugar and egg whites. Place over pan of barely simmering water; heat, whisking constantly, until a candy thermometer placed in the mixture registers 140 degrees. Remove from heat; whisk until cooled completely and stiff peaks form. Set aside. Whisk cream in medium bowl until stiff peaks form. Set aside in refrigerator.
In bowl of electric mixer fitted with whisk attachment, lightly whisk egg yolks until yolks are just broken up. In small saucepan, bring corn syrup to a boil; gradually and carefully pour hot syrup into egg yolks, whisking continuously. Continue to whisk until thick and fluffy, about 5 minutes. Fold the warm chocolate mixture into egg-yolk mixture. Fold into the reserved whipped cream, then gradually fold in egg-white mixture. Chill mousse in airtight container until ready to serve, up to 1 day.