For the tart casing:
Preheat the oven to 180ºC. Grease a 35cm x 12cm rectangular, fluted tart tin.
Place the 1/2 cup of chocolate and butter in a small microwave container and heat on the lowest setting until melted, stirring occasionally.
Add the cookie crumbs and orange zest to the chocolate mixture and blend in a food processor until well mixed.
Press mixture into the prepared tin. Bake for 15 to 20 minutes until set and lightly coloured. Cool completely.
For the filling:
In a small saucepan, heat the marmalade until warm to the touch and slightly runny.
Brush the bottom of the tart shell with some marmalade.
Place the chocolate in a medium bowl.
In a small saucepan over medium heat, heat the cream and 2 tablespoons (30ml) of the marmalade until small bubbles form around the edges of the pan.
Pour the hot cream over the chocolate.
Leave for 4 minutes, then whisk until smooth.
Pour the chocolate mixture into the casing.
Leave to cool for at least 2 hours. Remove from the fridge 5 minutes before serving.
Serve with the candied orange peel.