For the buttery biscuit base -
100g ginger snaps
175g bar of dark chocolate
Finely chopped hazelnuts (optional)
For the filling -
500g mascarpone cheese
1 tbsp icing sugar
1 punnet of blackberrys
Put the biscuits into a food processor and whizz until like breadcrumbs. Melt the butter on a low heat. When melted, add the biscuit crumbs and stir until all coated. Press the mixture evenly into a large tart or cake tin with a loose bottom. Place into the fridge to cool. In the meantime, take a pyrex bowl and break the chocolate up into pieces in it. Place a pan that will fit the base of the bowl into it comfortably onto the stove and heat water on a medium heat. Put the bowl onto the pan and stir chocolate until melted. This should take a few minutes so don't leave unattended.
Remove the base from the fridge, and pour on the chocolate. Tip the tin until the chocolate covers the whole base. As an optional extra, sprinkle finely chopped hazelnuts into the chocolate. Place back in the fridge for the chocolate to solidify. In the meantime, grate the zest and juice the limes. Beat the mascarpone with the icing sugar and add the zest and juice. Beat again until thoroughly combined. Wash the blackberrys. When the chocolate has set, scoop the cheese mixture on top and smooth over with a palette knife. Arrange the blackberrys in close knit rings around the whole of the circumference of the tart.
If you like, you can dust with icing sugar before serving for extra finesse.