Preheat oven to 425°F. Generously butter four 6-ounce ramekins and dust with cocoa powder, tapping out excess. Place ramekins on a baking sheet for easy handling.

In a microwave-safe bowl, combine 1/2 cup butter and chopped dark chocolate. Microwave in 30-second intervals, stirring between each, until completely melted and smooth. Let cool slightly for 2 minutes.

In a separate bowl, whisk together eggs, egg yolks, sugar, vanilla extract, and salt until thick and pale yellow, about 2 minutes. The mixture should slightly thicken when you lift the whisk.

Fold the chocolate mixture into the egg mixture until well combined. Gently fold in flour just until no streaks remain. Don't overmix. Divide batter evenly among prepared ramekins, filling each about 3/4 full.

Bake for 12 minutes until edges are firm but centers are still soft when lightly pressed. The tops should look set but the centers should still jiggle slightly. Don't overbake or centers won't be molten.

Let cakes cool in ramekins for 1 minute. Run a knife around edges and carefully invert onto serving plates. Let sit for 10 seconds before lifting off ramekins. Serve immediately with vanilla ice cream or whipped cream. The center should flow out when broken open.