Whisk the egg whites in a large bowl until soft peaks form when the whisk is removed.
Tip in the caster sugar and icing sugar and continue to whisk for 4-5 minutes more, or until stiff, glossy peaks form when the whisk is removed.
Whisk in the coffee essence, then set the mixture aside.
In a separate bowl, beat together the mascarpone, double cream and vanilla seeds until smooth and well combined.
Using the ring from the tart tin, stamp out the centres of the flan cases. Cut each disc of sponge in half horizontally to form two thinner discs (four in total).
Place the ring from the loose-bottomed tart tin onto a serving plate, then lay one sponge disc on top of it. Brush over one-quarter of the cold espresso, then spread over one-third of the mascarpone cream mixture.
Repeat the layering process with the remaining sponge discs, cold espresso, and mascarpone mixture, topping the cake with the final sponge disc.
Spoon the egg-white mixture on the cake and spread all over the top and sides in an even layer using a palette knife.
Using a chefs' blow torch, heat the meringue until crisp and light golden-brown.
To serve, dust the top of the gâteau with cocoa powder. Sprinkle over the marshmallows and crushed meringue shells, then garnish with grated dark chocolate. Dust with icing sugar just before serving.