Put the egg yolks into a bowl and whisk until light and fluffy (keep the whites for meringues). Combine the sugar and 4 fl ozs (120 ml/½ cup) cold water in a small heavy-bottomed saucepan. Stir over a gentle heat until the sugar is completely dissolved, then remove the spoon and boil until the syrup caramelises to a chestnut-brown. Quickly pour on 4 fl ozs (120 ml/½ cup) of hot water. Do not stir. Boil gently until it again becomes a smooth, thick syrup and reaches the ‘thread’ stage, 106°–113°C/223°–236°F. It will look thick and syrupy when a spoon is dipped in. Pour this boiling syrup onto the egg yolks. Add the vanilla essence and continue to whisk until it becomes a thick, creamy mousse.
Softly whip the cream: it should just hold the print of the whisk. Measure and make sure you have 1 pint (600 ml/2½ cups) of whipped cream. Fold the softly whipped cream into the mousse, pour into a bowl, cover and freeze.