Generously butter the insides of four 5 1/2-ounce ramekins.

Whisk egg yolks, eggs, and sugar together in a bowl with an electric mixer until foamy and lighter in color.

Melt chocolate and butter in a microwave-safe bowl in 30-second intervals until melted, stirring after each interval, 1 to 3 minutes.

Stir melted chocolate mixture into egg and sugar mixture until combined. Sift in cocoa powder and stir to combine. Sift in flour and salt; stir to combine into a batter. Stir in vanilla.

Transfer batter to a resealable plastic bag. Snip off one corner of the bag with scissors to create a tip. Divide batter evenly between the prepared ramekins; gently tap ramekins on the counter to remove any air bubbles. Refrigerate for 30 minutes.

Preheat the oven to 425 degrees F (220 degrees C).

Arrange chilled ramekins in a casserole dish. Pour enough warm tap water into the casserole dish to reach halfway up the sides of the ramekins.

Bake in the preheated oven for 15-18 minutes. Remove from the oven and let cool for 15 minutes.

Loosen the edges from each ramekin with a knife. Invert cakes onto serving plates and dust with powdered sugar.