Beat 6 egg whites and 12 egg yolks with the sugar for 15 minutes at low speed with an electric beater. Add the cream and vanilla. Heat to the boiling point. Let cool.
Melt enough brown sugar in a little water to line the bottom of a baking dish. Pour the batter over this and bake in a pan of hot water 1½ hours, or until set, in a preheated 250° oven.
Whip the other 6 egg whites with a little sugar until stiff but not dry. Spread over the dish of custard. Raise oven heat to 425°. Return baking dish to oven for 8 to 10 minutes, or until meringue is golden brown.