Butter two large soufflé dishes. (I used small copper pots) Melt chocolate and butter in heavy medium saucepan over medium-low heat until smooth. Transfer to large bowl and let it cool.
Using electric mixer, beat egg whites to soft peaks. Gradually add sugar; beat until stiff but not dry
Whisk yolks, flour and vanilla into melted chocolate mixture. Fold in 1/3 of egg whites to lighten. Fold in remaining whites.
Divide soufflé mixture evenly among soufflé dishes. Transfer to baking sheet. (Can be prepared 1 day ahead. Cover with plastic wrap and refrigerate.)
Bake soufflés until edges are set and a toothpick in the center comes out with wet batter attached, about 15 minutes if baked immediately or 20 minutes if refrigerated overnight.
Notes
When you crack the eggs, you'll separate the egg yolks and the egg whites in two bowls.You beat them on high while you gradually add in the sugar, and they will form soft peaks. This will be folded into the chocolate mixture.