Generously butter four 5 1/2 oz ramekins and place in a casserole dish.
Whisk egg yolks, eggs, and sugar until light, foamy, and pale yellow.
Melt chocolate and butter together in the microwave in 30 second bursts until smooth.
Stir the chocolate mixture into the egg mixture. Sift in cocoa powder, then flour and salt, stirring until smooth. Stir in vanilla.
Pipe the batter evenly into the ramekins and tap out any air bubbles.
Refrigerate for 30 minutes.
Preheat oven to 425°F (220°C). Place ramekins in a water bath halfway up the sides. Bake 15 to 18 minutes.
Cool 15 minutes, loosen edges with a knife, invert onto plates, and dust with powdered sugar. Serve immediately.