Egg whites must be brought to a temperature of 160° to be safe (we don't recommend eating raw eggs due to the risk of salmonella). This can be done without coagulating or cooking the whites (a no-no in a mousse) by combining them with sugar and beating over simmering water as we do here. The result is called an Italian meringue. If you don't have a double boiler, a bowl that fits over a saucepan will work fine.
Combine first 4 ingredients in a small saucepan. Cook over low heat until chocolate melts and mixture is smooth, stirring constantly. Remove from heat.
Combine sugar, 2 tablespoons water, cream of tartar, and egg whites in the top of a double boiler. Place over simmering water; beat mixture at high speed of a mixer until stiff peaks form (about 4 minutes). Gently stir one-fourth of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture and whipped topping. Divide mixture evenly among 4 dessert dishes. Cover and chill. Sprinkle each serving with 1/2 teaspoon grated chocolate.
Note: To freeze, cool completely; cover and freeze. Thaw in refrigerator.