Whisk the cream until it is really thick. In a separate bowl, take two of the egg whites and whisk them until they are thin, then add to the cream. Beat the four egg yolks thoroughly with the remaining egg whites, then whisk in the icing sugar, tablespoon by tablespoon, until it is thoroughly incorporated and the whole mixture is pale lemon and very frothy. Add the vanilla essence the add the egg mixture to the cream mixture, folding it in gently. Pour into a plastic container with a lid and place in a freezer for 4 to 6 hours until frozen solid.