Chop the dark chocolate into small pieces and melt together with the butter over a double boiler or in short bursts in the microwave, stirring until glossy and smooth. Let it cool slightly.
In a separate bowl, whisk the eggs and egg yolk with the sugar until the mixture is pale, thick, and slightly fluffy—about 2-3 minutes. This helps incorporate air for a tender crumb.
Gently fold the cooled melted chocolate and butter into the egg mixture, combining them until smooth and shiny. Be careful not to deflate the eggs.
Sift the flour and cocoa powder together and carefully fold them into the chocolate mixture, just until no streaks remain. The batter should be silky and pourable.
Butter the ramekins thoroughly and dust with cocoa powder, tapping out the excess. Chill the ramekins in the freezer for 10 minutes to help maintain their shape during baking.
Divide the batter evenly among the prepared ramekins, filling each about three-quarters full. Smooth the tops gently with the back of a spoon.
Place the ramekins on a baking tray and bake in a preheated oven at 200°C (390°F) for 12-14 minutes. The edges will be firm, and the tops may crack slightly, while the centers remain jiggly.
Remove the ramekins from the oven and let sit for 1-2 minutes. Carefully run a knife around the edges to loosen the sides.
Invert each ramekin onto a serving plate and gently tap to release the fondant. The molten center should ooze out as you lift it.
Serve immediately with a dusting of powdered sugar or a side of whipped cream for a perfect presentation and maximum molten indulgence.
