Step 1: Preheat your oven to 425°F (218°C) and grease two 6-ounce ramekins with butter, then dust them with cocoa powder, tapping out the excess.
Step 2: Melt the chopped chocolate and butter together using a double boiler or in short bursts in the microwave, stirring until completely smooth.
Step 3: In a medium bowl, whisk the eggs, egg yolks, sugar, and salt together until the mixture is slightly thickened and pale.
Step 4: Gently fold the melted chocolate mixture into the egg mixture until well combined.
Step 5: Sift the flour over the batter and fold it in using a spatula until no white streaks remain.
Step 6: Divide the batter evenly between the prepared ramekins.
Step 7: Bake for 12-14 minutes, or until the sides are firm but the center still jiggles slightly when shaken.
Step 8: Let the cakes cool in the ramekins for 1 minute, then carefully run a knife around the edges and invert them onto plates.