Preheat the oven to moderate 350°F (Gas Mark 4, 180°C).
With the butter, lightly grease two 7 inch loose bottomed sandwich tins and set them aside.
In a large mixing bowl, beat the egg whites with a wire whisk or rotary beater until they form stiff peaks.
Beat in 2 tablespoons of the sugar and continue beating until the peaks are very stiff and glossy. (You should be able to turn the bowl upside down without the mixture falling out.)
Using a metal spoon, fold in the remaining sugar.
Stir in the vanilla essence and fold in the ground hazelnuts.
Pour half the meringue mixture into each of the prepared tins and smooth with a flat bladed knife.
Place the tins in the oven and bake for 30 to 40 minutes or until the meringues are light gold and firm.
Remove the tins from the oven and allow the meringue cakes to cool in the tins for 5 minutes.
Remove the meringues from the tins, carefully transfer them to a wire rack and leave them to cool completely.
Meanwhile, place the chocolate and water in a heatproof bowl placed over a pan of hot water.
Set the pan over low heat and cook, stirring occasionally, until the chocolate has melted.
Remove the pan from the heat and the bowl from the pan.
Set aside to cool.
When the chocolate is cool but not set, using a metal spoon, fold in half of the cream and the toasted hazelnuts.
With a knife, spread the filling over one of the meringue cakes.
Place the other cake over the top.
Wash the knife and use it to spread the remaining cream over the top of the vacherin.
Arrange the apricot halves decoratively over the cream and serve immediately.