Cook 1/2 cup whipping cream and chocolate in a heavy saucepan over low heat, stirring constantly, until chocolate melts and mixture is smooth. Pour into a large bowl. Set aside.
Whisk together 2 cups whipping cream, yolks, 1/2 cup sugar, and vanilla in a bowl until sugar dissolves and mixture is smooth.
Whisk 1 cup egg mixture into chocolate mixture until smooth. Cover and chill remaining egg mixture. Pour chocolate mixture into 6 (8-ounce) custard cups; place cups in a 13- x 9-inch pan. Add hot water to pan to a depth of 1/2 inch.
Bake at 325° for 30 minutes or until almost set. (Center will be soft.) Slowly pour remaining egg mixture evenly over custards; bake 20 to 25 more minutes or until set. Cool custards in water in pan on a wire rack. Remove from pan; cover and chill at least 8 hours.
Beat remaining 1 cup whipping cream at high speed with an electric mixer until foamy; gradually add 3 tablespoons sugar, beating until stiff peaks form. Top custards with whipped cream; sprinkle with grated chocolate.