Start off by melting the dark chocolate and butter in a water bath. Melt the remaining butter in the microwave and grease the forms with a brush, then put each form in the freezer.
Beat the 4 eggs and 4 egg yolks with a mixer on high, add the sugar and continue to mix, add the melted dark chocolate and butter in several loads in a thin trickle and finally, add the flour that's been sifted several times but not all at once - do it gradually.
Remove the forms from the freezer and pour natural cocoa over them them, so that the cocoa sticks to the sides of the forms. Fill the forms with the mixture to one finger width below the rim. Bake in a preheated 392°F (200 °C) oven EXACTLY 12 minutes, no less, no more, it is important to obtain a liquid center.
If they're not going to be eaten immediately, put the soufles in the freezer. In this case you would have to bake them in the oven for 17 minutes – an increase in baking time by exactly 5 minutes.
Once ready, turn them over on a plate and sprinkle with powdered sugar.