Place the butter, chocolate, cocoa powder and salt in a bowl over a pan of simmering water and allow to melt slowly, stirring occasionally until well mixed.
In a separate bowl beat the eggs and sugar together until light and well creamed, and then add the golden syrup and sour cream or creme fraiche. Stir the chocolate mixture into this mixture, scraping all the chocolate out with a spatula. Once you?ve mixed it well, pour it into the pastry shell. Place into a preheated 150C/300 degree F/Gas 2 oven for 40 to 45 minutes. During cooking a beautiful crust will form on top.
Carefully remove the tart from the oven and allow to cool on a rack for at least 45 minutes, during which time the skin will crack and the filling will shrink slightly.