To make the crumble base, bash up the biscuits by hand in a Ziploc bag using a rolling-pin, or pulse in a food processor to get a rough textured crumble. Mix the instant coffee with the water and dissolve. Add this to the biscuit crumbs along with the milk and mix. Spread this on the base of a 28cm round serving (pie) dish or divide it amongst your individual serving dishes.
To make the parfait, dissolve the coffee in the hot water and heat this with the chocolate in a double boiler until melted. Allow to cool to a tepid temperature.
Separate the egg yolks from the whites and place the whites in a bowl. Using an electric beater whisk these until soft peak stage. Add the caster sugar slowly and then beat until the meringue is glossy.
In another bowl beat the cream until soft peaks.
Add the egg yolks to the chocolate mixture and using the beater you used to whip the cream, beat until well combined.
By hand gently fold the chocolate mixture through the egg whites and then through the cream. Fold this through so that it’s well combined without bashing out too much air.
Spoon the mousse into each glass and allow to set in the fridge.
Decorate with chocolate shavings or cream and chocolate shavings.