01 - Preheat the oven to 425°F (220°C). Generously butter four 6-ounce (175 ml) ramekins and lightly dust them with flour, tapping out any excess.
02 - In a heatproof bowl set over a saucepan of simmering water, melt the chopped bittersweet chocolate and 1/2 cup unsalted butter, stirring until the mixture is smooth. Remove from heat and allow to cool slightly.
03 - In a separate mixing bowl, whisk together the large eggs, egg yolks, powdered sugar, and pure vanilla extract until the mixture becomes thick and pale.
04 - Gently fold the cooled melted chocolate mixture into the egg mixture until thoroughly combined.
05 - Sift the all-purpose flour and a pinch of salt into the batter. Carefully fold until the dry ingredients are just incorporated, being careful not to overmix.
06 - Divide the prepared batter evenly among the four buttered and floured ramekins.
07 - Place the filled ramekins on a baking sheet and bake for 11–12 minutes, or until the edges of the cakes are set but the centers remain soft and molten.
08 - Allow the cakes to cool in their ramekins for 1 minute. Carefully run a knife around the inner edges of each ramekin, then invert the cakes onto individual serving plates. Serve immediately.