Preheat the oven to 425°F (220°C).
In a heatproof bowl, combine chocolate and butter and microwave for about 2 minutes, until fully melted. Stir every 15-20 seconds to prevent seizing. Set aside.
In a small bowl, stir eggs, egg yolks, and vanilla* to combine. Set aside.
In a medium bowl, whisk powdered sugar, flour, cocoa, and salt to combine. Set aside.
In a large mixing bowl, combine all ingredients. Using a rubber spatula or wooden spoon, slowly stir just until combined. Use a whisk to remove any lumps, if there are any. The batter will be thick. Set aside.
Butter and flour six 1-cup-sized (6oz / 240ml) ramekins.
Spoon batter evenly into the ramekins. Bake for 11-13 minutes. I baked mine for exactly 12 minutes. The edges will look solid and firm and the center soft. Remove from the oven and let stand 1 minute. Then place an inverted plate on top of each ramekin and turn over. Caution: the ramekins are still super hot! Use a napkin or oven mitt. Top with powdered sugar, ice cream, berries, or chocolate fudge sauce to your preference. Serve immediately.
Store leftovers in an airtight container in the fridge up to 1 day. Reheat in the microwave for about 20-30 seconds. However, they are best eaten fresh on the same day. Freeze up to 3 months.