Pour 5cm of water into a medium sized pot and bring to a simmer. Switch off the heat and place a bowl on top that fits snugly without touching the water. Break the chocolate into the bowl and leave to melt slowly, stirring occassionally.
Stir the coffee and 60ml of the cream into the chocolate until smooth. Remove from heat and leave to cool to room temperature.
In a seperate mixing bowl whip with an electric beater the Tatua Mascarpone, remaining 400ml cream and the sugar until thick enough to hold its shape.
Pour the chocolate mixture over the mascarpone mixture but do not stir. Place spoonfuls of the mascarpone mixture and chocolate evenly between 6 glasses to create a ripple effect. Serve immediately.