Beat together the cream cheese and caster sugar and stir in the chopped hazelnuts. Get a 7 in (18 cm) diameter fairly deep cake tin and line both the base and the sides with greaseproof paper or foil. Press and spread the cheese mixture about ¼ in (½ cm) thick over the bottom and up the sides approximately 2 in (5 cm) high. Cool in the fridge while you make the filling.
Break the chocolate into small pieces in a mixing bowl with 4 tablespoons of water. Put this bowl over a pan of gently simmering water and leave the chocolate to melt. Dissolve the gelatine and the brown sugar in a little water in a pan over a medium heat and stir into the melting chocolate. When it has melted turn the heat off and continue stirring for a few minutes and stir in the butter and rum. Remove the bowl from the pan. Separate the eggs putting the whites into a large bowl. Beat the yolks into the slightly cooled chocolate mixture. When the mixture feels almost cold whisk the egg whites until stiff and fold them gently into the chocolate mixture. Pour the mixture into the cream-cheese coating and leave to set in the fridge for an hour or more. If the cream-cheese sides come more than about ¼ in (½ cm) above the chocolate just push them down a bit. Melt the butter and stir it into the crumbled sweet biscuits. Spoon this mixture on top of the set chocolate mousse and press smoothly down with a metal spoon. Put in the fridge for another half hour or more. Turn the cake out of the tin on to a serving plate and carefully remove the greaseproof paper or foil. Grate a little chocolate on top if you like. Keep the pudding in the fridge until needed and serve with cream.