Start by making the gingerbread; pre-heat the oven to 180° C.
Grease and line a 2lb straight-sided loaf tin with nonstick baking paper. Sift together the flour, salt, ground ginger and bicarbonate of soda into a large mixing bowl. Cream together the butter and caster sugar in a separate mixing bowl using an electric hand-held whisk until pale and fluffy in appearance.
Beat in the Golden Syrup until incorporated, then beat in a couple of tablespoons of the flour mixture. Add the egg and beat well. Fold in the remaining flour mixture until incorporated, then follow with the milk until you have a smooth batter. Spoon into the prepared loaf tin and bake for 35-45 minutes until springy to the touch.
Remove and allow the gingerbread to cool in the loaf tin for 10 minutes before turning out. Allow to cool completely on a wire rack. Cut three-quarters of the gingerbread into 12cm x 5cm rectangular slices. Cut the remaining gingerbread into discs; one should be the diameter of the base of a Charlotte mould and another should be the same diameter as the top of the Charlotte mould. Cut any remaining gingerbread into discs to use in the filling.
Pre-heat the oven to 200° C.
Brush the slices of rectangular gingerbread and the discs with clarified butter, then position the disc of gingerbread for the base in the base of the mould. Arrange the rectangular pieces on the inside of the mould, overlapping them slightly if necessary, making sure there are no gaps.
Fold together the chestnut puree and chocolate together in a mixing bowl, then spoon into the centre. Place the disc of gingerbread for the top of the mould on top of the filling. Bake for 15-20 minutes until the gingerbread starts to become toasted. Remove from the oven and allow to sit in the mould for a few minutes before inverting onto a serving plate.
Garnish the top with the marron glacés (if using) before serving.