Preheat oven to 425°F (220°C). Grease 4 ramekins and dust lightly with cocoa powder.
Melt butter and chocolate together using a double boiler or microwave until smooth.
Whisk eggs, yolks, sugar, and vanilla in a bowl until pale and slightly thickened.
Combine mixtures: Slowly fold the melted chocolate into the egg mixture.
Add flour and salt, stirring gently until just combined.
Pour into ramekins evenly and place them on a baking tray.
Bake for 12–14 minutes, until the edges are firm but the center is still soft.
Invert onto plates while warm and serve immediately. Enjoy the molten chocolate center!