Whip the double cream until stiffish but definitely not
hard e.g. ~60 seconds on high speed in a Kenwood Chef.
Dollop some cream onto each ginger biscuit and assemble
concertina-style; probably best in a line, though in the above
it was doubled-up for space.
Dribble rum over the lot so that the ginger biscuits absorb
it.
Leave for a while (e.g. an hour) and repeat the dribbling;
like watering plants, you need to revisit for maximum
absorption.
If you want the biscuits to be well soaked, refrigerate
overnight and repeat the dribbling again in the morning.
With no rum residue on the plate, use a spatula to apply
the rest of the cream over the biscuits in a thin layer,
taking care not to remove too much soggy biscuit while doing
so.
Serve in very small slices; this is not for the feint
hearted.
