Thoroughly grease the inside of four ramekins with butter. In a bowl, beat the egg yolks, whole eggs, and sugar with an electric mixer until the mixture becomes pale and foamy.

Melt the chocolate and butter in the microwave in 30-second intervals, stirring after each interval until fully melted. Add the melted chocolate to the egg mixture and mix well. Sift in the cocoa powder and stir, then sift in the flour and salt and mix until smooth. Stir in the vanilla extract.

Transfer the batter into a resealable plastic bag and cut a small tip at one corner. Evenly divide the batter among the prepared ramekins. Gently tap them on the counter to remove air bubbles. Refrigerate for 30 minutes.

Preheat the oven to 425°F (220°C). Place the chilled ramekins into a baking dish and pour warm water into the dish until it reaches halfway up the sides of the ramekins.

Bake for 15 to 18 minutes. Remove from the oven and let cool for 15 minutes. Run a knife around the edges to loosen, invert onto serving plates, and dust with powdered sugar before serving.