In a medium bowl, whisk together the egg yolk and sugar until pale and fluffy. Add the mascarpone, vanilla pod seeds and double cream and whisk until the mixture is soft and thick.
Place the sponge biscuits in a shallow dish and pour over the coffee and Irish cream. Divide half the biscuits between 4 individual glasses.
Spoon half the mascarpone mixture over the biscuits and scatter with half the grated chocolate. Divide the remaining biscuits between the dishes and spoon over the remaining mascarpone mixture. Sprinkle over the rest of the chocolate and serve.