In small microwave-safe bowl, microwave the butter and dark chocolate together until melted, about 90 seconds. Add the biscuit crumbs and mix well to combine. Spoon into a 22cm fluted, round, loose-bottomed tart tin. Press the crumbs into the base and up the sides to form a lip then put into the fridge to chill slightly.
Whisk together the cream, eggs, orange zest and curd and pour into the slightly chilled base.
Bake in the oven for 30 to 35 minutes until just set. Remove from the oven and leave to cool then remove from the tin and drizzle with melted chocolate. Leave to cool completely and chill in the fridge before serving, about 90 minutes.
If you make your own orange curd, which contains orange zest, omit the zest from the recipe.