Combine egg yolks, dark brown sugar, and salt in a large mixing bowl; beat well. Gradually add whipping cream and half - and - half, stirring constantly. Stir in vanilla.
Pour custard into a 12- x 8- x 2-inch baking dish. Place dish in a 13- x 9- x 2 - inch baking pan; pour hot water into pan to a depth of 1 inch. Bake at 300° for 1 hour and 10 minutes or until a knife inserted between center and edge comes out clean. Remove baking dish from pan of water immediately. Cool to room temperature. Cover, and chill.
Just before serving, place custard in a 13- x 9- x 2-inch baking pan filled with ice. Sift light brown sugar evenly over custard. Broil 3 to 4 inches from heating element 2 minutes or until sugar melts and is caramelized. Remove custard from pan of ice. Spoon into individual serving bowls.