Preheat oven to 350 degrees.
Butter 9" springform pan. Line bottom of pan with parchment paper; butter paper. Wrap outside of pan with foil. Melt chocolate and butter in heavy saucepan over low heat until melted and smooth stirring constantly. Remove from heat. Cool to lukewarm, stirring often.
With electric mixer, beat egg yolks and 6 tablespoons sugar in large bowl until mixture is very thick and lemon colored, approx 3 minutes. Gently fold lukewarm chocolate mixture into yolk mixture. Fold in vanilla extract.
In a separate bowl and using clean dry beaters, beat egg whites until soft peaks form. Gradually add in remaining 6 tablespoons sugar, beating until medium-firm peaks form. Gently fold beaten whites into chocolate mixture in 3 additions. Pour batter into prepared pan.
Bake cake until top is puffed and cracked and tester inserted into center comes out with some moist crumbs attached, approx. 50 minutes. Cool cake in pan on rack.
Using knife, cut around the sides to loosen cake. Remove pan sides. Invert cake onto rack. Peel off parchment paper.
Glaze- In medium saucepan bring cream and corn syrup to simmer. Remove from heat. Add chopped chocolate and whisk until melted and smooth.
Place cake (still on rack) on a baking sheet, covered with foil, to catch the drippings. Spread 1/2 cup glaze over top and sides of cake. Freeze for approx 3 minutes until almost set. Remove from freezer and pour remaining glaze over cake; smoothing sides and top. Place cake on platter. Chill for approx 1 hour- until glaze is set. Serve at room temperature.