Place the butter, chocolate, and a small pinch of sea salt in a heatproof bowl over a pan of simmering water. Sift in the cocoa powder, then allow everything to melt slowly, stirring occasionally until well mixed.
In a separate bowl, beat the eggs and sugar together until light and well creamed, then add the golden syrup and soured cream.
Stir your 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.
Bake 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 wire rack for at least 45 minutes, during which time the skin with crack and the filling will shrink back slightly.