Preheat the oven to 180'C.
Cut the butter into small cubes and mix together with the icing sugar and salt until crumbly. Sift in the flour and cocoa powder and add the egg.
Using your fingers, rub the flour into the butter until you have a breadcrumb consistency. Remove the mixture form the bowl and work it until you have a dough. Knead the pastry gently until it is smooth and then wrap in clingfilm and allow to rest for 30 minutes in the fridge.
To line the tart case: Soften the butter and coat the inside of the tart case with the butter and then dust with flour.
After 30 minutes, roll the pastry into a circle large enough to line an 8 inch loose bottomed tart case. The pastry should be about 3mm thick. Line the tart case, allowing any excess to hang over the edges of the case and then blind bake in the oven for 20 minutes. Remove the lining and bake for a further 5 minutes.
Remove from the oven and allow to cool.
Reduce the oven temperature to 165'C.
Place the chocolate into a mixing bowl. Mix the eggs with the sugar. Bring the milk and double cream to the boil in a saucepan and pour onto the chocolate. Mix well until the chocolate is melted. Add the eggs to the mixture using a whisk.
Pour the filling into the tart case and then bake in the oven for 12-15 minutes or until the filling is just set. Remove from the oven and allow to cool. The filling will continue to set as it cools. Once the filling is set, trim off any excess pastry with a serrated knife.
Remove the skin from the oranges with a sharp knife making sure to remove all the white pith. Remove the segments by cutting alongside either side of the membrane. Place the segments in a bowl. Cut the peel into strips and then place in a saucepan with the sugar and water. Boil until the peel is soft and the water and sugar have reduced to a syrup.
Remove from the pan and dust in caster sugar.
Whisk together the cream, icing sugar and Grand Marnier until thick.
Remove the tart from the tart case and cut a generous wedge. Serve with a few segments of orange, some candied peel and a quenelle of the cream.
«When making this tart use good quality dark chocolate such as Valrhona.»