Step 1: Preheat your oven to 425°F (218°C) and generously butter two 6-ounce ramekins, then dust them with cocoa powder, tapping out the excess.
Step 2: In a heat-proof bowl, combine the chopped chocolate and butter. Melt them together using a double boiler or in the microwave in 30-second bursts, stirring until smooth.
Step 3: In a medium bowl, whisk together the whole eggs, egg yolks, sugar, salt, and vanilla extract for about 2 minutes until the mixture is light and slightly thickened.
Step 4: Slowly pour the melted chocolate mixture into the egg mixture, whisking constantly to ensure the eggs don't scramble.
Step 5: Gently fold in the flour using a spatula just until no white streaks remain. Do not over-mix.
Step 6: Divide the batter evenly between the prepared ramekins. Place them on a baking sheet and bake for 12-14 minutes, or until the sides are firm but the center still jiggles slightly.
Step 7: Remove from the oven and let rest for 1 minute. Carefully run a thin knife around the edge and invert onto a plate to serve immediately.