Mix the flour, salt and sugar in a bowl. Place the chocolate and margarine in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate while stirring, continue to stir until smooth. Stir in the vanilla paste and apple sauce. Mix into the dry ingredients, transfer the cake mixture to the prepared tin.
Bake for approx. 10 mins. in the centre of an oven preheated to 250°C.
Remove from the oven, allow to cool slightly, remove from the tin and leave to cool completely on a rack. Dust the cake with cocoa powder.