For the baked Alaska, bring 50ml/5fl oz water and the sugar to the boil in a saucepan. Keep boiling until the mixture reaches 121C on a sugar thermometer. (Caution: Hot sugar can be dangerous as it boils over very quickly. Do not leave unattended.)
Meanwhile, whisk the egg whites in a large clean bowl, using a freestanding mixer or an electric whisk, until stiff peaks form when the whisk is removed.
When the sugar reaches 121C, remove the pan from the heat. With the whisk or mixer running slowly, carefully pour the sugar syrup into the whisked egg whites in a thin stream, taking care not to let the hot sugar hit the beaters as it may splash. Continue to whisk until all the syrup has been incorporated into the egg whites and the mixture is glossy.
Continue to whisk the egg white mixture at a slow speed until it has almost completely cooled (about 5 minutes).
For the chocolate sauce, heat the chocolate with 2 tablespoons water in a saucepan over a low heat. When the chocolate has melted, add half of the cream and bring the mixture to the boil, whisking continuously.
When the mixture is boiling, remove the pan from the heat and whisk the mixture until smooth. Whisk in the remaining cream until combined. Decant into a serving jug and set aside until needed.
To assemble the baked Alaska, place the sponge disc onto a heatproof serving plate and drizzle the Armagnac all over it. Upturn the tub of ice cream in the centre of the sponge base and cut away the packaging.
Spread half of the meringue mixture all over the ice cream and sponge base using a palette knife. Transfer the remaining meringue mixture to a piping bag with a star nozzle and pipe it over the top of the meringue layer to decorate.
Using a chefs’ blow torch, heat the piped meringue all over until it caramelises to a light golden-brown. Alternatively, heat your oven to its highest setting and place the baked Alaska into the oven for a few seconds.
Serve immediately with the chocolate sauce alongside.