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.