Pre-heat the oven to 200°C. Brush 4 200ml ovenproof ramekins with some of the melted butter, making sure to use upwards strokes with the pastry brush. Chill the ramekins for 15 mins then brush with another layer of melted butter.
Dust the insides with the cocoa powder, tapping lightly to get rid of any excess. Chill until ready to fill with the batter. Prepare the batter by melting together the chocolate and cubed butter in a heatproof mixing bowl set over a pan of gently simmering water. Once melted, remove from the heat and allow to cool for 10 minutes.
Meanwhile, whisk together the eggs and egg yolks in a separate mixing bowl until light and thick. Sift the plain flour into the eggs and whisk until smooth. Pour the melted chocolate and butter mixture into the egg and flour mixture in thirds, mixing well between each addition. Divide the fondant batter evenly between the chilled moulds and arrange on a baking sheet.
Bake in the oven for 10-12 minutes until the tops are set and the edges are starting to come away from the sides of the ramekins. Remove from the oven and allow to sit for 1 minute before carefully running a knife around the edges to loosen the fondants. Carefully tip into your hands. Arrange on serving plates and dust with icing sugar before serving.