Pour the hot milk and the Grand Marnier over the gingerbread biscuits, cover and leave to soften for approx. 30 mins. Blend until smooth using a hand blender, set aside. Melt the chocolate in a thin-sided bowl over a gently simmering bain-marie, stir until smooth, then stir into the gingerbread mixture. Dissolve the gelatine in the boiling water, mix together with 2 tbsp of the gingerbread mixture and immediately stir thoroughly into the remaining mixture. Chill until the mixture is just set at the edges. Stir the mixture until smooth, fold in the whipped cream. Cover the mousse and chill for approx. 1 1/2 hrs.
To serve & decorate
Plate up the mousse, decorate with the gingerbread biscuits.