Preheat oven to 450 °F.
In a double boiler, melt the chocolate and butter until glossy and smooth.
In a medium bowl, whisk eggs, egg yolks, sugar, salt, and vanilla until light and fluffy.
Let the chocolate mixture cool slightly, then add it to the egg/sugar mixture until smooth.
Fold in the flour with a spatula until no streaks remain.
Coat the inside of your ramekins with a thin layer of butter and dust lightly with cocoa powder. Shake out any excess.
Spoon the batter into your ramekins, 2/3 of the way full.
Bake for 12 - 16 minutes until the cakes have a very slight sinkhole appearance in the middle.
Let the cakes cool slightly and de-mold.