Grease and line three baking trays, and draw out 3 circles roughly 10 inches (25 centimeters) in diameter (I use a cake tin as my guide and just trace around it with a pencil) on the parchment paper.
Whisk the egg whites until soft peaks begin to form, then add the sugar spoonful by spoonful.
Once you have added all the sugar and the eggs become stiff, gently fold in the cornstarch and the vinegar.
Spoon the meringue on to the baking trays, dividing it equally between the three and spreading it out to make three discs.
Bake in the oven for 1 hour, then switch the oven off and leave the meringues in there to harden for another hour. You want the meringue to be crisp so that it can support the weight of the cream.
Sift the confectioners' sugar into the cream and whip until it becomes stiff. Set the first meringue disc on a serving dish and top with a third of the whipped cream, and a third of the marrons. I like to crumble them over the cream.
Roughly chop the chocolate and sprinkle half of it over the marrons.
Top with the second meringue disc and repeat the process again.
Top with the third meringue disc and cover with whipped cream.
Decorate with sprigs of fresh thyme and the last of the crumbled marrons.