How to Make It
Preheat the oven to 350° F. Grease twelve 4-ounce ramekins or aluminum foil molds with the softened butter. Dust with cocoa powder and tap out excess.
Melt the chocolate and butter in the top of a double boiler, or a heatproof bowl, set over barely simmering water, stirring occasionally until smooth. Remove from the heat.
In a large bowl, whisk together the eggs, egg yolks and sugar until well mixed. Whisk in the melted chocolate and butter. Sift the cake flour over the top and fold it in.
Divide the batter among the ramekins, filling them no more than three quarters full. Arrange them on a baking sheet and bake for 5 to 6 minutes. Turn the baking sheet around and bake for another 4 to 6 minutes, just until the edges of the cakes look set and have pulled slightly away from the sides of the ramekins but the centers are still slightly liquid; do not overbake.
Turn each cake out on a plate and serve immediately.
Use high-quality bittersweet chocolate for this recipe, such as Valhrona or Callebaut, available at specialty shops, or Lindt, which can be found in most supermarkets.
The batter can be prepared ahead and the unbaked cakes frozen, well wrapped, for a few hours, or as long as overnight. Bake the frozen cakes for 9 minutes, turn the baking sheet around, and bake for another 9 minutes or until the edges of the cakes look set and have pulled slightly away from the sides of the ramekins but the centers are still slightly liquid; do not overbake.