• Place the double cream, sugar and a tiny pinch of sea salt in a pan and bring to the boil.
• Break up the chocolate.
• As soon as the mixture has boiled, remove from the heat and add the butter and chocolate. Stir until it has completely melted. Allow the mixture to cool slightly, stirring in the cold milk until smooth and shiny.
• Sometimes this mixture looks like it has split. Allow to cool down a bit more and whisk in a little extra cold milk until smooth.
• Scrape all the mixture into the cooked and cooled pastry shell with a spatula.
• Shake the tart to even it out and allow to cool for around 1 to 2 hours until it is at room temperature. Dust with the cocoa powder.
• Ultimately the pastry should be short and crisp and the filling should be smooth and should cut like butter.

Pastry Shell

• In the bowl of a standing mixer fitted with the paddle, mix 1 cup of the flour with the salt. At low speed, add the butter pieces, a handful at a time. When all of the butter has been added, increase the speed to medium and mix until the butter is completely incorporated. Reduce the speed to low and add the remaining 1 cup of flour just until blended. Mix in the water just until thoroughly incorporated. Flatten the pastry into an 8-inch disk, wrap in plastic and refrigerate until chilled, at least 1 hour or overnight.
• Set the ring of a 9-inch springform pan on a rimmed baking sheet lined with parchment paper, leaving the hinge open. Brush the inside of the ring with oil.
• Dust the pastry on both sides with flour. On a lightly floured work surface, roll out the pastry to a 16-inch round, about 3/16 inch thick. Carefully roll the pastry around the rolling pin and transfer to the prepared ring, pressing it into the corners. Trim the overhanging pastry to 1 inch and press it firmly against the outside of the ring. Use the trimmings to fill any cracks. Refrigerate the shell for 20 minutes.
• Preheat the oven to 180°. Line the pastry shell with a 14-inch round of parchment paper; fill the shell with dried beans or rice. Bake for about 40 minutes, or until the edge of the dough is lightly browned. Remove the parchment and beans and continue baking the pastry shell for about 15 minutes longer, or until richly browned on the bottom. Transfer the baking sheet to a rack and let the pastry cool. Fill any cracks with the reserved pastry dough.