Rub the butter into the flour and salt until it resembles breadcrumbs, add the water and bring it together into a dough. Press into a 20 cm tart tin, working it up the sides, then chill for 20 minutes.
Prick the base all over with a fork, line with baking paper and baking beans or dried rice, and bake blind at 180C (160C fan) for 12 minutes. Remove the paper and beans and bake 5 minutes more until pale gold and dry.
Meanwhile, pour the evaporated milk into a large bowl. Add the muscovado sugar and whisk with an electric hand whisk for 5 to 8 minutes until the mixture is thick, mousse-like and roughly doubled in volume. By hand with a balloon whisk it needs a good 15 minutes.
Pour into the warm case and level the top. Bake for 30 to 35 minutes until the filling is set at the edges with a slight wobble in the middle.
Cool completely in the tin before slicing. The filling firms up as it cools and cuts much more cleanly cold. Cut into 8 slim slices, it is very sweet.