Preheat the oven to 200C/400F/Gas 6. Grease a 30cm/12in tart tin.
For the pastry, place the flour, cocoa powder and caster sugar in a food processor, add the butter and pulse until the mixture resembles breadcrumbs. Add the milk and blend again until the mixture just comes together. Remove the pastry from the food processor and press it into the bottom and sides of tart tin.
Line the tart case with tin foil, fill with baking beans and bake blind for 10 - 15 minutes. Remove the baking beans and foil, cover the edge of the tart case with foil to prevent burning, and return to the oven for 10-15 minutes. Remove the tart case from the oven and set aside to cool.
For the mocha filling, melt the chocolate in a microwave or in a bowl suspended over a pan of barely simmering water. Whisk together the egg yolks, sugar and cornflour until pale and creamy.
Heat the milk and coffee powder until just simmering, then whisk into the egg mixture until well combined. Pour the mixture into the saucepan and cook over a gentle heat, stirring all the time, until the mixture has thickened enough to coat the back of a spoon. Whisk in the melted chocolate, then pour the mixture into the pastry case and transfer to the fridge for 1-2 hours.
For the meringue, place the egg whites, salt and lemon juice in the bowl of a food mixer, whisk slowly until frothy, then turn up the speed and whisk until the mixture holds stiff peaks. Mix the granulated sugar and egg white powder together then gradually add to the egg whites, whisking all the time, until the mixture is smooth and glossy.
Heat the treacle, sugar and water in a small pan to 110C/225F then pour into the egg whites in a thin stream, whisking until the bowl is cool and the meringue mixture is stiff.
Spread a thin layer of meringue over the surface of the pie to cover the filling completely then pipe the remaining meringue on top. Using a blow torch, lightly brown the meringue and serve immediately.