Crush and sprinkle finely crushed Ameretti biscuit over the base of a 9 inch (23cm) spring form tin.
Put the chocolate, glucose and rum in a bowl and melt over a pan of water, remembering to stir frequently. Once it has all melted and mixed take off the heat and allow to cool for a few minutes until warm.
Beat the cream until it has slightly thickened and fold in the chocolate mix a bit at a time ensuring it is mixed evenly. Spoon the mixture into the tin and spread evenly. Cling film the top of the tin and leave to set in the fridge overnight.
Run a palette knife around the edge of the tin and remove from the tin. Dust the top with coco powder and serve.