Preheat your oven to 200°C (390°F). Gently butter four ramekins and dust them with cocoa powder, tapping out the excess to prevent sticking.
In a heatproof bowl set over simmering water, melt the dark chocolate and butter together, stirring until smooth and glossy. Remove from heat and let cool slightly.
In a separate bowl, whisk the eggs and superfine sugar together until light and slightly frothy, about 1 minute, then stir in the vanilla extract.
Pour the cooled chocolate mixture into the egg mixture, whisking constantly to combine the silky ingredients evenly.
Sift the cocoa powder into the batter and fold gently with a spatula until fully incorporated, avoiding deflating the mixture too much.
Add the flour and fold just until the batter is smooth, with a slightly thick, velvety texture.
Divide the batter evenly among the prepared ramekins, filling each almost to the top for a generous molten center.
Place the ramekins on a baking sheet and bake for 10–12 minutes. The tops should be set and slightly cracked, with the centers still jiggly when gently shaken.
Remove the fondants from the oven and let them rest for 2 minutes. The crust will be firm but delicate, with a tempting crack revealing the molten interior.
Gently invert each ramekin onto a serving plate using an offset spatula or small spoon, allowing the rich chocolate to flow out in a velvety pool.
Serve immediately while warm, with a dusting of cocoa or a scoop of vanilla ice cream if desired, enjoying the contrast of crispy crust and molten center.
