Step 1
Preheat oven to 375 degrees F (190 degrees C).
Step 2
Grease a 9-inch springform pan and dust with 1/2 teaspoon unsweetened cocoa powder.
Step 3
Melt chopped chocolate squares and butter in a heavy saucepan over low heat stirring until smooth 5 to 10 minutes.
Step 4
Whisk egg yolks and vanilla extract together in a large bowl.
Step 5
Gradually whisk chocolate and butter mixture into egg yolk mixture until well blended.
Step 6
Beat egg whites in a separate bowl with an electric mixer on high speed until soft peaks form. Gradually beat sugar into egg whites until sugar dissolves and egg whites hold stiff peaks.
Step 7
Fold 1/3 of the egg white mixture into chocolate mixture. Gently fold in remaining egg white mixture until just blended and batter is smooth and fluffy.
Step 8
Spread batter evenly into prepared springform pan.
Step 9
Bake in the preheated oven until sides begin to pull away from pan and top is set in center about 25 minutes.
Step 9
Let torte cool in pan on a wire rack for 10 minutes before removing the sides of pan.