One orange will make plenty of candied orange peel. Leftovers will stay crunchy for a few days and can be served in bowls as a snack for festive nibbling. Try dipping leftovers into dark chocolate for a delicious chocolate orange hit.
Preheat oven to 180C/350F/Gas 4.
For the chocolate mousse, bring a little water to the boil in a pan, then reduce the heat until the water is simmering. Suspend a heatproof bowl over the water (do not allow the base of the bowl to touch the water). Add the butter and dark chocolate and stir until melted, smooth and well combined. Remove from the heat and set aside to cool slightly.
Whisk the egg yolks and sugar in a bowl until nearly white and thick in consistency.
Gently stir the whisked yolks into the butter and chocolate, then add the coffee.
Whisk the egg whites until stiff peaks form when the whisk is removed, then gently fold them into the chocolate mixture, until just combined.
Transfer the mousse to small cups or ramekins and chill in the fridge for 1-2 hours, or overnight.
For the ginger shortbread, cream the butter until pale and fluffy in a bowl. Sift in the flour, icing sugar and cornflour and beat again until fully incorporated. Stir in the vanilla essence and grated root ginger.
Place walnut-sized balls of the mixture onto a greased baking tray, about 2cm/¾in apart (you should get about 18 from this mixture). Spread each ball of the biscuit mixture out slightly using the back of a spoon, until flattened into biscuit shapes. Top each ball with a slice of crystallised ginger, then transfer to the oven and bake for 12-15 minutes, or until the shortbread biscuits are pale golden-brown. Remove from the oven and set aside to cool for a few minutes, then carefully lift them off the tray and onto a wire rack, using a palette knife, and set aside to cool completely.
For the candied orange peel, fill a small saucepan with water and bring to the boil. Add the orange peel and bring to the boil again, then drain well.
Re-fill the pan with water, bring to the boil, return the orange peel to the pan, return the water to the boil, then drain again.
Re-fill the pan with 150ml/5fl oz of water, add the caster sugar and bring to the boil, stirring well. Add the orange peel, reduce the heat until the mixture is simmering and simmer for 15-20 minutes, or until the liquid resembles syrup, stirring occasionally.
Tip the demerara sugar into a shallow bowl.
Once the syrup mixture has thickened, remove the orange peel and coat each piece in the demerara sugar. Shake the peel in the sugar until it is completely covered. (NB: Once cool, keep the candied orange peel uncovered until needed.)
To serve, place the ramekins of chocolate mousse onto serving plates. Place one or two shortbreads alongside each. Sprinkle the mousse with the candied orange peel.