Preheat your oven to 200°C (390°F). Lightly butter four ramekins and dust them with cocoa powder, tapping out the excess.
Chop the dark chocolate into small pieces and melt it together with the butter in a heatproof bowl set over simmering water. Stir until smooth and glossy, then set aside to cool slightly.
In a separate bowl, whisk the eggs, sugar, and a pinch of salt until the mixture is frothy and slightly increased in volume, about 1 minute.
Pour the cooled chocolate mixture into the egg mixture and gently fold together with a spatula until fully combined, maintaining a smooth, shiny batter.
Sift the flour and cocoa powder into the batter and fold gently until just incorporated, being careful not to deflate the mixture.
Divide the batter evenly among the prepared ramekins, smoothing the tops with a spatula and running a finger around the edges to prevent sticking.
Place the ramekins on a baking tray and bake in the preheated oven for 10-12 minutes. The edges should be set, but the center still jiggly when gently shaken.
Remove the ramekins from the oven and let them rest for 1-2 minutes. Carefully invert each onto a serving plate and gently tap to release.
Serve immediately while the molten center is flowing, optionally dusted with cocoa powder or powdered sugar, and with your favorite accompaniments like berries or whipped cream.
