Step 1

Grease a 9-inch springform pan, and dust with unsweetened cocoa powder.

Step 2

Melt chopped chocolate and butter in a heavy saucepan over low heat, stirring until smooth.

Step 3

Whisk together egg yolks and vanilla in a large bowl. Gradually stir in warm chocolate mixture, whisking until well blended.

Step 4

Beat egg whites at high speed with electric mixer until soft peaks form. Gradually add sugar, beating until sugar dissolves and stiff peaks form. Fold one-third beaten egg white mixture into chocolate mixture; gently fold in remaining egg white mixture just until blended. Spoon batter into prepared pan, spreading evenly.

Step 5

Bake at 375 degrees for 25 minutes. (Do not overbake!) Let stand in pan on a wire rack 10 minutes before removing sides of pan