For the baked Alaska, place the water and sugar into a pan and bring to the boil. Skim the surface and wipe the sides of the pan with a wet pastry brush to remove any sugar crystals, then increase the heat so that the syrup boils rapidly.
Insert a sugar thermometer. When the sugar mixture reaches 110C/225F beat the egg whites with an electric mixer until stiff peaks are formed.
When the sugar mixture reaches 121C/250F remove from the heat. It will have formed a syrup.
Turn the electric mixer onto the lowest setting then pour the sugar syrup into the egg whites in a thin, steady stream, continuing to beat until all the syrup is incorporated.
Continue to beat at a slow speed until the mixture is almost cold, this can take up to 15 minutes.
Fold the hazelnuts into the egg white mixture then spoon into a piping bag fitted with a plain nozzle.
Place the chocolate sponge onto a serving plate and top with alternate scoops of vanilla and toffee ice cream.
Pipe the meringue around the ice cream, covering completely from top to bottom. Glaze the meringue with a mini-blowtorch, or bake briefly in a very hot oven.
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, cut wedges of baked Alaska and place on serving plates. Drizzle the sauce over the top.
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