Make the espresso. Once cool, add the alcohol, stir, transfer into a shallow bowl and set aside.
Pour the cream into a bowl*. Using an electric whisk, whip until it forms stiff peaks. Using a silicone spatula, gently fold in the vanilla extract.
Break the chocolate and place into a heatproof bowl. Sit over a pan of simmering water (a bain marie) and allow the chocolate to melt. Once melted, fold in 3 tablespoons of the whipped cream until combined.
Dip four digestive biscuits in the espresso and rum mixture, one at a time then transfer into a glass, two in each one.
Divide the chocolate mixture between the two glasses, then place in the fridge for 5-10 minutes to set.
Dip the remaining four digestive biscuits in the espresso and rum mixture, and place on top of the chocolate mixture, two in each glass.
Top with the remaining whipped cream - you can do this with a spoon or transfer the cream into a piping bag and pipe the cream for a neater pattern.
Place in the fridge and chill for at least 30 minutes.
To serve, dust with cocoa powder.