Line a 20 cm long loaf tin with parchment or baking paper and set aside.
Melt the dark chocolate in a bowl over a pan of simmering water.
Add 8 teaspoons of water to the chocolate and mix until smooth and glossy.
Add the mixed spice and leave to cool.
Whip the cream until soft peaks form.
Gently fold the whipped cream into the cooled chocolate mixture. Set aside.
Slice 5 Bonne Maman Madeleines horizontally and use them to cover the bottom and sides of the tin.
Mix the remaining 7 teaspoons of water with the rum, then use a pastry brush to soak the Madeleine slices.
Cover the Madeleine layer with about half the chocolate and cream mixture.
Cover this layer with 5 more horizontally sliced Bonne Maman Madeleines.
Spoon the chestnut cream evenly over this layer of Madeleines and then carefully cover with the rest of the chocolate mixture.
Place another layer of 5 horizontally sliced Madeleines on top, press down well. Cover with cling film (plastic wrap) and place in the fridge overnight to set.
Turn the log onto a serving plate, garnish with remaining Madeleines and dust with icing sugar and any decorations you like.