Preheat your oven to 200°C (392°F). Lightly butter your ramekins and dust them with cocoa powder to prevent sticking.
Chop the dark chocolate into small pieces. Gently melt it over a bain-marie or in short bursts in the microwave, stirring until smooth and glossy.
In a separate bowl, soften the butter and mix it with the melted chocolate until fully combined. Let it cool slightly so it doesn't scramble the eggs.
Add the eggs one at a time to the chocolate mixture, whisking vigorously after each addition until the batter becomes glossy and slightly thickened.
Stir in the caster sugar and vanilla extract, whisking until smooth and shiny.
Sift the flour into the batter and gently fold it in with a spatula, just until no streaks remain. Be careful not to overmix to keep the batter airy.
Divide the batter evenly among the prepared ramekins, filling each about three-quarters full. Tap the ramekins lightly on the counter to release any air bubbles.
Place the ramekins on a baking tray and bake in the preheated oven for exactly 12-14 minutes. The edges should be set and firm, but the center should still jiggle when gently shaken.
Remove the ramekins from the oven and let them sit for 2-3 minutes. The crust will firm slightly while the center remains molten and flowing.
Run a small knife around each ramekin's edge to loosen the fondant, then carefully invert onto serving plates. Gently lift the ramekin to reveal the gooey center.
Serve immediately while warm, perhaps dusted with a little cocoa powder or garnished with fresh berries for an elegant touch. Enjoy that luscious, flowing center with each spoonful.
