Preheat the oven to 450F. Butter and lightly flour 6 (4- to 5-ounce) ramekins and place them on a baking sheet.
Combine the butter and chocolate in a double boiler and melt over low heat, stirring until smooth. Or, place the butter and chocolate in a microwave-safe bowl and microwave on high, stirring every 30-seconds until melted. Whisk together until smooth.
In a large mixing bowl, combine the eggs, egg yolks, sugar, and salt. Beat on medium speed until thick and pale yellow. Fold the chocolate mixture and flour into the egg mixture until well combined. Divide the batter among the prepared ramekins.
Bake for 6 to 8 minutes or until the sides of the cakes are firm set but the center is jiggly. Let the cakes cool in the ramekins for 1 minute.
Place a small dessert plate over the top of each ramekin and carefully turn it over, inverting the cake onto the plate. Let stand for about 10 seconds, then remove the ramekin to unmold the cakes. Sift powdered sugar on top and garnish with berries, if desired. Serve immediately.
