Preheat the oven to 180C/360F/Gas 4.
Grease four ovenproof coffee or tea cups (or ramekins) with butter.
Place the dark chocolate and the butter into a heatproof bowl placed over a pan of barely simmering water (make sure the water doesn't touch the bottom of the bowl).
Stir the chocolate and butter mixture until melted and combined, then remove from the heat and allow to cool slightly.
Beat the eggs in a separate bowl until foamy, then add the sugar and beat until pale and thick.
Gradually add the melted chocolate and butter mixture to the eggs, stirring continuously.
Sift the flour, baking powder and cocoa powder together, then fold into the chocolate and egg mixture.
Pour the mixture into the prepared cups and bake for 8-12 minutes (depending on the size of the cups), or until cooked but still gooey in the middle.
For the sauce, melt the sugar in a heavy-based pan over a medium heat, stirring continuously.
Add the butter and whisk well until combined, then remove from the heat and whisk in the cream.
To serve, place the puddings in their cups onto small serving plates. Pour over the caramel sauce and finish with a dollop of whipped or clotted cream.