Preheat oven to 350. Butter 9 inch spring form pan. Line bottom of pan with parchment paper. Stir chocolate and butter in heavy medium saucepan over low heat until melted and smooth. Remove from heat. Cool to lukewarm. Using electric mixer, beat egg yolks and 6 tablespoons sugar in large bowl until mixture is very thick and pale, about 3 minutes. Fold lukewarm chocolate mixture into yolk mixture, then fold in vanilla extract. Using clean dry beaters, beat egg whites in another large bowl until medium-firm peaks form. Gradually add remaining 6 tablespoons sugar, beating until medium-firm peaks form. Fold 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, about 50 minutes. Cool cake in pan on rack. Using small knife, cut around pan sides to loosen cake. Remove pan sides. Invert cake onto tart pan bottom. Peel off parchment paper. Glaze: Bring cream and corn syrup to simmer in medium saucepan. Remove from heat. Add chocolate and whisk until melted and smooth. Place cake on rack set over baking sheet. Spread 1/2 cup glaze smoothly over top and sides of cake. Freeze until almost set, about 3 minutes. Pour remaining glaze over cake, smooth sides and top. Chill until glaze is firm, about 1 hour. Garnish with chocolate shavings. Serve at room temperature.