Put the crushed biscuits in a bowl and pour on the melted butter and orange zest. Mix to combine.
Press the biscuit crumb base into the mini tart tins (making sure the biscuit extends up the sides to create a hollow for the ganache filling). Place in the fridge for 1 hour to chill (or 30 minutes in the freezer).
Place the chopped chocolate orange and dark chocolate into a heatproof bowl.
Put the double cream into a small saucepan and bring to a simmer (do not let it boil!), then quickly pour the cream onto the chopped chocolate. Leave for 3 minutes.
Use a metal spoon to gently and slowly stir the ganache together. Leave the ganache to firm up a bit before assembling the tarts (15-20 minutes in the fridge should be enough to make the ganache easier to spoon into the tarts).
Take the tart tins out of the fridge or freezer and unmould then onto a serving plate.
Spoon in the chocolate ganache and decorate with a chocolate orange mini segment.
Chill for at least 1 hour (or overnight) before serving.