If you are using a Miele steam oven, melt the chocolate along with the butter for about 20 minutes with the steam cooking function at 100°C.
Otherwise, you can place the chocolate and butter in a heatproof bowl set over a pan of simmering water. While with the Miele steam oven there is no need to stir the mixture continuously, in the alternative method you will have to keep stirring with a spatula until the chocolate and butter completely melt and transform into a glossy, creamy consistency.
Set the mixture aside to cool off while preparing the rest of the ingredients.
Whip the eggs, sugar and vanilla for about 2 minutes. Then, pour the liquid chocolate mixture into the eggs and mix well. Finally, sift the flour and the baking powder and add them to the mixture.
Stir well with a spatula. Cover with cling film and leave the dough in the fridge for a minimum of 4 - 5 hours, preferably 12 hours.
The dough must be chilled and solid-like. Scoop the dough with a spoon and roll into balls, about 18g of dough per ball. Roll each ball in granulated sugar, then generously in the icing sugar.
Place them on a baking tray keeping a distance of about 6 cm between each other and store them in the fridge for another 20 minutes. Chilling is necessary before baking them.