Grease a 23cm (9in) pie dish, or fluted tart tin, and line it with pastry. Line with a piece of nonsitck baking paper inside, fill with baking beans and bake blind for 10 minutes while you make the filling.
Melt the chocolate and the butter in a bowl over simmering water, stirring occasionally. As soon as it starts to melt, remove it from the heat and stir. As soon as there are no lumps left fold in half of the crème fraîche. In a separate bowl whisk the egg yolk, the whole eggs and the sugar until they form a thick, white, frothy cream. Pour the chocolate mixture in, folding it through very thoroughly but without knocking too much air out.
Pour into pastry case and cook for 30-40 minutes at Gas Mark 3, 160°C, fan 140°C until the middle has set.
Lightly whip the remaining crème fraîche with the orange liqueur, and serve with the tart, along with some orange curls.