Preheat the oven to 180C/365F/Gas 4.
Place the butter and chocolate into a heatproof bowl. Place the bowl over a pan of barely simmering water, and allow the chocolate and butter to melt.
Once the chocolate and butter have melted, stir to combine, then set aside and allow to cool.
Place the eggs and sugar into a clean bowl and whisk for 4-5 minutes, or until pale and light.
Add the egg mixture to the chocolate mixture and gently fold together, taking care to maintain volume in the eggs.
Add the flour and cocoa powder and fold in.
Grease and flour six 200ml/7fl oz ramekins or dariole moulds.
Divide the chocolate mixture among the ramekins. Place onto a baking tray and transfer to the oven to bake for 7-8 minutes, or until set, but still with some wobble in the centre.
Loosen the puddings and very carefully turn each out onto plates.
Dust the puddings with cocoa powder and serve with a dollop of mascarpone on top.