Melt the dark chocolate with the butter in a bain-marie. Whisk eggs, sugar and vanilla then add the melted chocolate and mix well. Mix in the flour with a pinch of salt and bake in a 150C oven until the centre is still soft. Cool down completely before serving.
To make the mousse, juice the apples and mix with the lemon juice and sugar then cook until reduced by two thirds and refrigerate. In a bowl, whisk the mascarpone cheese with the chilled apple juice and the vanilla seeds until mousse like and serve.