Preheat oven to 350°. Butter six 3/4 cup souffle dishes or custard cups. Dust lightly with cocoa powder.
Stir chocolate and butter in heavy medium saucepan over medium-low heat until melted and smooth; cool.
Using electric mixer, beat eggs, egg yolks, sugar and salt in large bowl at high speed until pale yellow and thick, about 5 minutes.
Fold cooled chocolate into egg mixture. Sift flour over; fold until well incorporated. Divide batter equally among prepared dishes. Transfer to baking sheet. Bake until set at edges, but still soft in the center, about 18 minutes.
Before serving, run a sharp knife around the edge of the ramekin. Flip over onto a serving dish, tap a few times until the cake fall out. Serve warm with real whipped cream or vanilla ice cream.
TIP: Take the cake out of the oven when the uncooked portion in the middle is the size of a quarter.
NOTE: The batter can be made ahead the day before and stay in the refrigerator until you are ready to bake it. Cooking time varies greatly depending on how cold or room the batter is.