Slice the cake into half-inch layers; cut the cake layers in such a way that you can form a large oval 1/2 - inch thick and 1- inch larger all around than your ice cream mold. Arrange the oval on the platter, cover airtight with plastic wrap, and set aside. Be sure oven is preheated to 450 degrees in time for step 2. Beating the meringue: Start beating the egg whites at moderate speed until they are foaming, beat in the salt and cream of tartar, then gradually increase speed to fast until egg whites form soft peaks. Gradually beat in the sugar, sprinkling in 1/4 cup at a time, beating half a minute between additions. After all has gone in, add vanilla, and continue beating for several minutes at high speed until eggs form stiff, shiny peaks. Set aside ice-cream mold in a basin of tepid water, run a knife around edge, and unmold the sherbet or ice cream upon the cake in the platter. Immediately spread the meringue over it with a spatula, starting at bottom of cake and bringing it up to a peak at the top of the dessert. Meringue should be about 1- inch thick over the sherbet or ice cream, in order to insulate it from the heat of the oven. Sieve confectioner's sugar all over surface of meringue, making a layer about 1/16 -- inch thick. Browning: Set in upper-middle level of preheated 450 degree oven for 3 to 4 minutes, to brown meringue lightly. Just before serving, place a sparkler on top of the baked Alaska and light.