Preheat oven to 180°C.
Melt butter and dark chocolate in a heatproof bowl over simmering water until smooth.
Whisk egg yolks and caster sugar until thick and pale.
Stir the yolk mixture into the melted chocolate and add the espresso shot.
Whisk egg whites to stiff peaks and gently fold into the chocolate base.
Divide mousse into ramekins and chill for at least 1 hour.
Cream butter for shortbread, then beat in flour, icing sugar, cornflour, vanilla, and grated ginger.
Place small balls of dough 2cm apart on a tray, flatten slightly, top with ginger, and bake for 12-15 minutes.
Boil orange peel in water twice, draining between each boil.
Simmer peel with 150ml water and caster sugar for 15 minutes until syrupy.
Coat the warm peel in demerara sugar and allow to cool.
Serve mousse with biscuits and topped with candied peel.