Preheat the oven to 200°C and grease a 30cm tart tin.
Pulse flour, cocoa, sugar, and butter in a processor, then blend in milk to form pastry.
Press pastry into the tin and bake blind with foil and beans for 10-15 minutes.
Remove beans and foil, cover edges, and bake for another 10-15 minutes before cooling.
Melt the chocolate and whisk egg yolks, sugar, and cornflour until pale.
Simmer milk and coffee, whisk into eggs, and cook until thickened.
Stir in melted chocolate, pour into pastry case, and chill for 1-2 hours.
Whisk egg whites, salt, and lemon juice to stiff peaks.
Mix sugar with egg white powder and whisk into whites until glossy.
Heat treacle, sugar, and water to 110°C and whisk into egg whites.
Spread and pipe meringue over the tart, brown with a blowtorch, and serve.