Preheat the oven to gas 6, 200°C, fan 180°C. Lightly butter 6 egg cups and dust with a little caster sugar.
Put the remaining butter in a bowl with the chocolate and heat over a pan of gently simmering water, stirring until the chocolate melts. Remove from the heat, allow to cool a little then beat in the egg yolks.
Whisk the egg whites in a clean bowl until they form stiff peaks. Add the remaining caster sugar and whisk until thick, glossy peaks form.
Fold into the chocolate mix then divide the mixture between the prepared egg cups, leaving 1cm (½in) of space at the top of each cup.
Bake for 8-10 minutes, until well risen and puffed up. Dust with a little icing sugar and cocoa powder mixed together and serve at once.