Put the flour, butter and Speculoos in a food processor and blitz to crumbs. Add the sugar and pulse again, then add the egg yolk and 1 tbsp iced water. Blitz until the pastry comes together in a ball. You may need to add the other 1 tbsp water if it is still crumbly. Press into a round, wrap in clingfilm and chill for 30 minutes.
Preheat the oven to 180°C/gas mark 4. When the pastry has chilled, dust your work surface with icing sugar and roll out the pastry into a large disc, big enough to line a 22cm loose-bottomed tart case. Press the pastry into the tin and prick the base with a fork. Line with a sheet of baking paper and fill with baking beans. Bake in the oven for 15-20 minutes or until the pastry has set. Remove the beans and bake for another 5 minutes. Leave to cool.
For the filling, put the dark chocolate and butter in a heatproof bowl. Heat the cream and sugar until steaming, then pour over the chocolate. Stir until the chocolate and butter have melted and you have a smooth, glossy ganache. Pour into the cooled tart case. Chill for an hour or so, until the ganache has set.
To make the mallow, add the gelatine and 100ml water to a small saucepan to dissolve. Put the egg whites in the bowl of a freestanding mixer, with the whisk attached.
Place the sugar and liquid glucose in a pan with 100ml water and gently heat, stirring until dissolved. Bring to the boil, leaving a sugar thermometer in the syrup to check the temperature. When the syrup reaches 115°C, start whisking the egg whites. When the syrup reaches 120°C, remove from the heat and place the gelatin and water mixture in the other saucepan over a very low heat (making sure it does not boil). Slowly pour the sugar syrup into the egg whites while whisking continuously, then pour the melted gelatin in and continue whisking until cold, and the mallow is soft but holds its shape.
Put the mallow in a piping bag with a round nozzle attached and pipe the mallow on top of the pie. Use a blowtorch to slightly scorch the outside, then serve.