Sieve flour with salt twice. Place the chocolate pieces along with the margarine in a basin, place the basin on the top of a saucepan with barely simmering water and wait for them to melt. Add the sugar and stir until it melts. Remove basin from saucepan. Add the eggs, one at a time, and whisk briskly until they are completely incorporated. Add the rum, vanilla essence and the flour, stirring constantly, until you get a smooth mixture. Finally add the coarsely chopped pistachio nuts. Take a large, oblong shaped cake mould, grease it well with butter and dust it with cocoa powder. Alternatively you can use a round shaped mould with a hole in the middle. Pour the mixture in. Bake at 250ºC, in bottom shelf, for exactly 5 minutes. Turn heat off and leave the cake in for 15 minutes, without opening the oven door. Then remove the cake from the oven, let it rest for 10 minutes and then turn it out on to a serving plate. If you slice it while warm, you will see the liquid chocolate seeping out. When it goes completely cool the slices look moist. Serve with whipped crème fraîche or ice cream. Dust it with icy sugar and sprinkle with pistachio nuts.