Preheat the oven to 180°C fan-assisted.

To make the sweet pastry, place the butter and sugar into a food processor and blitz until creamed. Add the egg yolks and blitz until well combined. Add the flour and blitz to form fine bread crumbs, then add the water. Blitz for a few seconds until the pastry just comes together to form a ball. Remove from the processor and gently combine to create a smooth pastry without overworking it. Wrap in cling film and refrigerate for 30 minutes.

Grease a Le Creuset 28cm Fluted Flan Dish. Lightly dust your work surface with flour and roll out the pastry to ½ cm thick once chilled. Line the dish by pressing the pastry into all the grooves. Trim the excess pastry and poke holes into the base using a fork. Place a piece of baking paper over the tart and fill with blind baking beans (alternatively, rice or dried beans). Refrigerate for 30 minutes. Blind bake for 40 minutes. After 40 minutes, remove the beans and paper and bake for a further 20 minutes until slightly golden. Remove from the oven and allow to cool.

Reduce the oven temperature to 160°C fan-assisted. Place the butter, chocolate, and a small pinch of sea salt in a heatproof bowl over a pan of simmering water to make the filling. Sift in the cocoa powder, then allow everything to melt slowly, stirring occasionally until well mixed. In a separate bowl, using a hand beater, beat the eggs and sugar together until light and well creamed, then add the golden syrup and sour cream. Beat until combined. Beat the chocolate mixture into the egg mix. Pour into the cooled tart shell and bake for 50 minutes until set. Remove from the oven and allow to cool completely before serving. Serve with fresh berries and ice cream (optional).