Preheat your oven to 200°C (390°F). Generously grease four ramekins with butter and dust with cocoa powder to prevent sticking.
Chop the dark chocolate and melt it together with the butter in a heatproof bowl set over a saucepan of simmering water. Stir gently until the mixture is smooth, glossy, and fragrant with a rich chocolate aroma. Remove from heat and let cool slightly.
In a separate bowl, whisk the eggs, egg yolk, and sugar together using a whisk until the mixture becomes pale, slightly thickened, and frothy—about 2 minutes. This creates a light, airy base for the batter.
Gently fold the melted chocolate mixture into the egg mixture in three additions, using a spatula to keep the batter airy and smooth. Carefully incorporate the flour and vanilla extract if using, folding until just combined.
Divide the batter evenly among the prepared ramekins, filling each about three-quarters full. Smooth the tops with a spatula for an even surface.
Place the ramekins on a baking tray and bake in the preheated oven for 12-14 minutes. The edges should be set but the center still jiggles slightly. The aroma will be intensely chocolatey and inviting.
Once baked, remove the ramekins from the oven and let them rest for about 1 minute. Carefully invert each ramekin onto a serving plate, gently tapping or shaking to release the molten fondant.
Allow the fondants to sit for about 10 seconds, then lift the ramekin away. The rich, gooey center should flow out with a satisfying crackle, revealing a glossy, molten core.
Serve immediately, perhaps with a dusting of cocoa powder or a scoop of vanilla ice cream to enhance the indulgence.
