Slather butter on the inside of 4 ramekins (5 1/2 oz), and place in a casserole dish.
Whisk the eggs and sugar in a bowl until lighter in colour and foamy.
Melt chocolate and butter (microwave or double boiler, or on med-low heat stirring constantly).
Stir chocolate into egg mixture until well combined.
Sift the cocoa into the mixture and stir well.
Sift the flour and salt into the mixture and stir well. This will form a batter.
Add the vanilla, stir another 20 seconds.
Transfer to a piping (or Ziploc) bag.
Pipe the batter into the ramekins, and gently tap them on the counter to remove any air bubbles.
Refrigerate 30 minutes.
Preheat oven to 425F
Place the chilled ramekins in a casserole dish.
Pour hot water (tap, not boiling) to reach halfway up the sides of the ramekins.
Bake for 15-18 minutes, let cool 15 minutes.
Carefully separate the cake from the ramekin by running a knife along the inside.
Invert each cake onto a plate, and dust with powdered sugar.