Prepare the Ramekins: Begin by greasing six 4- to 5-ounce ramekins thoroughly with butter. Lightly dust each ramekin with flour to prevent sticking, tapping out any excess. Arrange the ramekins on a baking sheet to make handling easier during baking.
Melt Butter and Chocolate: Using a double boiler, gently melt the butter and chopped bittersweet chocolate together over low heat. Stir continuously until the mixture is completely smooth and glossy. Alternatively, you can melt the butter and chocolate in short bursts in the microwave, stirring every 30 seconds until fully combined.
Whip Eggs and Sugar: In a large mixing bowl, whisk the whole eggs, egg yolks, sugar, and salt using an electric mixer on medium speed. Continue beating until the mixture becomes thick, creamy, and pale yellow in color, indicating enough air has been incorporated for a tender cake.
Combine Chocolate and Flour: Carefully fold the melted chocolate mixture into the whipped eggs. Then sift the flour over the batter and gently fold it in just until no streaks remain. Avoid over-mixing to keep the batter light and airy.
Fill and Bake: Evenly divide the batter into the prepared ramekins, filling each about three-quarters full. Place the baking sheet with ramekins into a fully preheated oven at 450°F (232°C). Bake for 6 to 8 minutes, watching closely—the edges should be set and firm while the centers remain soft and slightly wobbly.
Unmold and Serve: Once baked, let the cakes cool in their ramekins for about one minute. Then, place a dessert plate on top of each ramekin and carefully invert to release the cakes. Allow them to sit briefly, then lift off the ramekins. Dust with powdered sugar and garnish with fresh berries if desired. Serve immediately for the best molten effect.