Preheat your oven to 200°C (390°F). Lightly butter four ramekins and dust them with cocoa powder to prevent sticking.
Chop the dark chocolate into small pieces and melt it together with the butter in a double boiler or heatproof bowl over simmering water, stirring gently until smooth and glossy, filling your kitchen with that rich cocoa aroma.
In a separate bowl, whisk the eggs and caster sugar vigorously for about 2 minutes until the mixture is pale, slightly thickened, and aerated, creating a light, fluffy texture.
Pour the melted chocolate and butter into the egg mixture, gently folding together with a spatula until just combined, keeping the batter smooth and shiny.
Sift in the flour and cocoa powder, then fold carefully until the batter is thick, velvety, and uniform, being cautious not to deflate the mixture.
Divide the batter evenly among the prepared ramekins, smoothing the tops with a spatula for an even rise during baking, then place them on a baking tray for stability.
Bake the fondants in the preheated oven for 10–12 minutes. The tops should crack slightly, and the edges will be firm while the center remains soft and jiggly.
Once baked, let the fondants rest for about 1 minute, then carefully run a knife around the edges to loosen them.
Invert each ramekin onto warm plates, gently tapping or shaking to release the fondant, revealing the glossy, molten interior that starts to flow out.
Serve immediately, perhaps with a dusting of cocoa powder or a scoop of vanilla ice cream, to enjoy the contrast of the crisp exterior and luscious, gooey center.
