Preheat your oven to 200°C (390°F). Generously butter each ramekin and dust with cocoa powder, tapping out the excess to prevent sticking and help achieve a shiny finish.
Chop the dark chocolate into small pieces, then melt it together with the butter in a heatproof bowl over simmering water, stirring until smooth and glossy. Remove from heat and let cool slightly.
In a separate bowl, whisk the eggs and caster sugar together until the mixture becomes pale, frothy, and slightly increased in volume, about 2 minutes. Add a splash of vanilla extract or a pinch of sea salt if desired.
Gently fold the melted chocolate and butter into the egg mixture using a spatula, being careful not to deflate the airy batter. Sift in the flour and fold just until combined, maintaining a light texture.
Divide the batter evenly among the prepared ramekins, filling each about three-quarters full. Smooth the tops with the back of a spoon to ensure even baking.
Bake the fondants in the preheated oven for exactly 12 to 14 minutes. The edges should be set and firm, while the center still jiggles slightly when gently shaken—this is key to that molten core.
Once baked, remove the ramekins from the oven and let them rest for about 1 minute to settle. Carefully run a small knife around the edges to loosen them, then invert each onto a serving plate.
Let the fondants rest for about 30 seconds before gently lifting the ramekin off, revealing a shiny, cracked surface with a luscious, flowing center.
Serve immediately, optionally dusted with cocoa powder or powdered sugar, paired with ice cream or whipped cream if desired, to enjoy that perfect molten chocolate experience.
