For the ganache, heat the cream until it just begins to simmer and then pour this over the chopped chocolate. Let this sit a minute, then gently stir with a spatula starting at the centre and widening the circles until the chocolate has fully melted and the ganache is smooth. Chill until firm, about 2 hours.
Preheat the oven to 425°F. Grease four 5-ounce ramekins, coat them with sugar, shaking out any excess, and place them on a baking tray.
Melt the butter and chocolate in a metal bowl placed over a pot of gently simmering water, stirring until smooth. Remove this from the heat (but it can still be warm).
Whip the eggs, egg yolks and sugar until they have doubled in volume and hold a ribbon when the beaters are lifted, about 3 minutes. Fold in the melted chocolate, then sift in the cocoa powder and fold in (using the beaters). Divide this batter between the 4 ramekins and chill for 15 minutes.
Spoon the chilled ganache into 4 truffles (you can roll the truffles between your palms to shape) and gently place this one into the centre of each of the cakes, pressing down into the batter slightly, but not to the bottom.
Bake the molten cakes for about 9 minutes, until the top of each cake has domed up a bit and has a dull finish. Let the cakes sit for at least 2 minutes. Lift each up using a towel, run a spatula around the cake to loosen, place a plate overtop and invert, and lift off the ramekin. Garnish with a dusting of icing sugar and serve immediately.
Alternately, the batter can be prepared, ramekins filled and then chilled until ready to bake and serve. Simply add an additional 2 minutes to the baking time.