Chop the dark chocolate into small pieces and melt it gently in a heatproof bowl set over simmering water or in short bursts in the microwave, stirring until smooth. Set aside to cool slightly.
Gently melt the butter and mix it into the cooled melted chocolate until fully combined, creating a glossy, smooth mixture.
In a separate bowl, whisk together the eggs and caster sugar until the mixture becomes pale, slightly thickened, and has a gentle sheen, about 2-3 minutes.
Pour the cooled chocolate and butter mixture into the egg mixture, folding gently to combine, ensuring a smooth, shiny batter.
Sift the cocoa powder and flour into the batter, then fold carefully until just combined. The batter should be thick, shiny, and pourable.
Grease four ramekins with butter and dust with cocoa powder to prevent sticking. Divide the batter evenly among the ramekins, filling each about two-thirds full.
Place the ramekins on a baking sheet and bake in a preheated oven at 200°C (390°F) for 10-12 minutes. The edges should be set but the center still jiggles gently when shaken.
Remove the ramekins from the oven and let rest for 1-2 minutes. Then, carefully invert each ramekin onto a serving plate, giving it a gentle tap to release the fondant.
Serve immediately while warm, showcasing the glossy shell and molten center that spills out as you cut into it.
