1. Generously butter the inside of 4 (5 1/2 ounce) ramekins. Place them in a casserole dish.
2. Whisk together egg yolks, eggs, and sugar in a bowl until light, foamy, and lemon colored.
3. Melt chocolate and butter in a microwave-safe bowl in 30-second intervals, stirring after each melting, 1 to 3 minutes.
4. Stir melted chocolate mixture into egg and sugar mixture until combined.
5. Sift cocoa powder into the mixture; stir to combine.
6. Sift flour and salt into the mixture; stir to combine into a batter.
7. Stir vanilla extract into the batter.
8. Transfer batter to a resealable plastic bag. Snip one corner of the bag with scissors to create a tip.
9. Divide batter evenly between the prepared ramekins; tap gently on the counter to remove any air bubbles.
10. Refrigerate 30 minutes.
11. Preheat an oven to 425 degrees F (220 degrees C).
12. Arrange the ramekins in a casserole dish. Pour enough hot tap water into the casserole dish to reach halfway up the sides of the ramekins.
13. Bake in the preheated oven for 15-18 minutes. Set aside to cool for 15 minutes.
14. Loosen the edges from the ramekin with a knife. Invert each cake onto a plate and dust with powdered sugar.