source: Jamie Oliver
for one tart
Place cream, sugar and a pinch of salt in a pan and bring to a boil. 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. Add the cold milk and stir until mixture is smooth and shiny. Sometimes this mixture looks like it has separated. Allow it 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. Shake the tart to even it out and allow it to cool for 1-2 hours until it is at room temperature. Dust with the cocoa powder. Ultimately the pastry should be short and crisp and the filling smooth and cut like butter.
Needs to rest a while in the fridge (1 hr plus), otherwise it will remain runny.