Preheat oven to 180ºC.
Whisk together the butter and icing sugar until light and creamy. Zest the peel from the orange and finely dice.
Add to the mix with the egg and whisk to combine.
Sift the salt and flour together and fold in with the drinking chocolate to make a soft dough.
Wrap in plastic wrap and refrigerate for 1 hour.
Roll out the pastry to 2-3mm thickness and line a rectangular tart shell 35cm x 13cm with the pastry.
Refrigerate for another 30 minutes then blind bake for 10-15 minutes until pastry is cooked. Allow to cool.
Chop the chocolate and place into a large bowl.
Bring the cream to the boil and pour over the chocolate.
Stir the mixture until smooth and pour into the cooled tart shell. Refrigerate until chocolate is set.
To serve, remove the tart from the tin and use a warm knife to slice. Serve with candied orange zest, fresh orange segments and fresh cream.