Preheat your oven to 200°C (390°F). Generously butter four ramekins and dust them with cocoa powder, tapping out the excess so they’re well coated.
Chop the dark chocolate into small pieces and melt it together with the butter in a heatproof bowl over simmering water, or in short bursts in the microwave, stirring until smooth and glossy. Let it cool slightly to prevent cooking the eggs.
In a separate bowl, whisk the eggs and sugar together until just combined and slightly frothy, about 30 seconds. This helps incorporate some air and creates a tender crumb.
Pour the slightly cooled chocolate mixture into the egg mixture, gently folding with a spatula until smooth and uniform. Be careful not to deflate the batter.
Add the flour and cocoa powder to the batter, folding gently until just combined. This small amount of flour stabilizes the center but keeps it gooey inside.
Divide the batter evenly among the prepared ramekins, filling each about three-quarters full. Tap each ramekin lightly on the counter to release any air bubbles and to level the surface.
Bake in the preheated oven for 12 to 14 minutes. The edges should be set and firm, but the center will still jiggle slightly when gently shaken—this is key to that molten core.
Remove the ramekins from the oven and let them sit for about 1 minute. Then, carefully invert each onto a serving plate, gently lifting the ramekin to release the fondant. The tops should crack open slightly, revealing the gooey interior.
Serve immediately while warm, with an optional dusting of cocoa powder or a scoop of vanilla ice cream. The contrast of the crisp exterior and molten center makes each bite a delight.
