Lightly grease the inside of 4 ramekins with soft butter (do not use melted butter. Soft butter provides a thicker coating). Sprinkle 1 tablespoon castor sugar into each ramekin. Rotate and shake the ramekin to completely coat the inside. Turn the ramekin upside down and tap gently to remove excess sugar. Preheat oven to 200°C.
Put chocolate and butter in a heatproof bowl over a saucepan of simmering water. Heat stirring with a metal spoon for 5 minutes or until mixture is smooth. Take the saucepan away from the heat.
Beat eggs and sugar until pale and thick using an electric mixer. Add warm chocolate mixture.
Sift flour over the mixture and stir gently to combine. Pour batter evenly into the ramekins.
Bake for 2025 minutes or until pudding tops are cracked. Leave the pudding to stand in the ramekins for 45 minutes. Carefully loosen the puddings and turn each upright onto a plate.
For the sauce: Combine chocolate and whipping cream in a small saucepan. Heat over low heat, stirring with a metal spoon until sauce is smooth. Drizzle warm sauce over the pudding and serve at once.