Preheat the oven to 425°F. Spray four 6-ounce ramekins with cooking spray and place them on a baking sheet.
Add the butter and chocolate to a microwave-safe bowl. Microwave on high for one minute, or until the butter is melted. Remove the bowl from the microwave and stir until the chocolate is completely melted.
Add the powdered sugar and stir until combined, then add the eggs, egg yolks, and vanilla, and stir until smooth. Add the flour, and stir until combined. Divide the batter evenly between the ramekins.
Bake for 13 minutes, then remove from the oven and let stand for 5-10 minutes. Run a plastic knife around the edge of each cake to release it from the ramekin, then invert it onto a plate.
Dust with powdered sugar and serve warm.