Melt the butter in a small pan and stir in the crushed biscuits. Lightly press into the base of a 23cm/9in deep loosed-bottomed fluted flan tin. Chill whilst preparing the filling.
To prepare the filling: place the lime juice into a large bowl, add the cream and condensed milk. Whisk for 1-2 minutes. Add the lime zest and lightly stir. Pour onto the prepared biscuit base. Place onto a tray and chill in the fridge for 1-2 hours.
Decorate with crystalized lime zest and serve.