Make the pastry. Place the icing sugar and flour in a bowl, add the butter and rub in with your fingertips until the mixture resembles fine breadcrumbs. Add the egg, orange rind and salt and mix to a soft dough. Wrap in clingfilm and chill for 20 minutes.
Roll out the pastry on a lightly floured work surface and use to line a 23 cm (9 inch) tart tin. Chill for 30 minutes. Trim off the excess pastry, then line the tart with baking paper and fill with baking beans. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes. Remove the paper and beans, then cook for a further 10 minutes until crisp and golden. Leave to cool. Reduce the oven temperature to 180°C (350°F), Gas Mark 4.
Melt the chocolate and butter in a heatproof bowl set over a small pan of simmering water, making sure the water does not touch the surface of the bowl. Leave to cool slightly. Beat the eggs, egg yolks and sugar in a bowl until light and fluffy, then stir in the cooled chocolate mix.
Spread the marmalade over the tart, then pour in the chocolate mixture. Bake in the oven for 15–20 minutes until just set. Leave to cool, then decorate with sliced Crystallized Orange (see below). Serve with crème fraîche.