For the meringue, preheat the oven to 100C/200F/Gas ½. Line two baking trays with baking parchment.
Place the egg whites into a bowl and whisk until soft peaks form when the whisk is removed. Gradually add the sugar and continue to whisk until stiff and glossy. Fold through the roasted chopped hazelnuts.
Spoon the mixture into a piping bag fitted with a small plain nozzle. Draw 18 x 7.5cm/3in diameter circles onto the baking parchment on the baking trays. Pipe the meringue in a spiral to fill each circle.
Transfer to the oven and cook for 20 minutes, then turn the oven off but leave the meringues in there for at least two hours. When crisp, remove from the oven and allow to cool completely.
For the coffee and praline cream, put the sugar in a pan and heat. Cook until the sugar has melted and caramelised to a golden-brown colour.
Add the nuts to the caramel and swirl to combine and coat the nuts in caramel.
Pour the caramel and nut mixture into a well-greased, lined baking tray and leave to cool and set. Once set, remove the mixture and break up into small pieces.
Place the cream in a large mixing bowl and whisk until soft peaks form when the whisk is removed. Add the vanilla, coffee, icing sugar and nearly all of the praline, leaving a little for the top, and mix.
For the toffee sauce, pour the cream into a pan and heat gently. Add the sugar and butter and whisk until melted. Whisk in the syrup and treacle.
To serve, place a meringue disc onto each of six serving plates. Spread a little of the praline and cream mixture over them and then some sauce. Top with another meringue and the remaining cream and some sauce, and then finish with a third meringue. Garnish with the remaining praline and the remaining sauce.