Preheat and prepare: Preheat oven to 450°F (230°C). Generously butter six 4-5 ounce ramekins and lightly dust with flour, tapping out excess. Place ramekins on a baking sheet.
Melt chocolate mixture: Combine butter and chopped chocolate in a double boiler and melt over low heat, stirring until smooth. Alternatively, place in a microwave-safe bowl and microwave on high in 30-second intervals, stirring between each interval until completely melted and smooth.
Prepare egg mixture: In a large mixing bowl, combine whole eggs, egg yolks, granulated sugar, and salt. Beat with an electric mixer on medium speed for 2-3 minutes until mixture becomes thick and pale yellow in color.
Combine ingredients: Add the melted chocolate mixture to the egg mixture and gently fold together. Sprinkle flour over the mixture and fold until just combined. Do not overmix.
Fill and bake: Divide batter evenly among prepared ramekins, filling each about 3/4 full. Bake for 6-8 minutes, or until the sides are firm and set but the center still jiggles slightly when gently shaken.
Cool and unmold: Remove from oven and let cool in ramekins for exactly 1 minute. Place a small dessert plate over each ramekin and carefully invert. Let stand 10 seconds, then gently lift off the ramekin. Dust with powdered sugar and garnish with berries if desired. Serve immediately while the center is still molten.
