Melt the 20g butter and use to brush the inside of the 5 dariole moulds. Sift some cocoa powder into each and shake it around so the insides of the moulds are lightly dusted. Tap out any excess and place the moulds in the freezer. Cut the butter into cubes and break the chocolate into pieces. Put the butter and chocolate into a medium heatproof bowl. Bring a pan of water to the boil, then take it off the heat. Stand the bowl over the pan, making sure the base of the bowl is not touching the water. Leave to melt, stirring occasionally. Put the eggs, extra yolks and sugar in a large bowl and whisk using a hand-held electric whisk until thick and mousse-like. This can be done over a steaming pan to speed up the process, but must be whisked off the heat until cool again. Using a large metal spoon, fold the melted chocolate mix into the egg mixture, then sift and carefully fold in the flour. Divide between the chilled moulds, filling them to 1cm from the top. Refrigerate for at least 30 minutes, or up to 24 hours. Heat the oven to 200C/gas mark 6. Place the fondants on a baking tray and cook in the oven for 12-15 minutes, depending on the oven (an electric oven will be quicker than a gas oven). Remove the spare 'tester' fondant after 12 minutes to test. It should be well risen and set on the outside but still molten in the centre when you cut into it. When they are ready, take the remaining puddings out of the oven. Leave them to stand for a minute or two, then remove the fondants from the moulds, inverting them onto plates. Serve immediately.