Adapted from Bon Appetit
12 ounces semisweet chocolate, chopped
Chocolate shavings or berries to garnish, optional
Preheat oven to 350º. Grease 9 inch springform pan. Line bottom of pan with parchment paper; grease top of parchment. Wrap outside of pan with foil (I did this but not sure why it is necessary!).
Microwave butter and chocolate together until melted and smooth, stopping at 30 second intervals to stir. Cool to lukewarm, stirring often.
Using electric mixer, beat egg yolks and 6 tablespoons sugar in large bowl until mixture is very thick and pale, about 3 minutes. Fold chocolate into yolk mixture, then stir in vanilla. Using clean dry beaters, beat egg whites in another large bowl until soft peaks form. Gradually add remaining 6 tablespoons sugar, beating until firm peaks form. Carefully 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...beware,cake will fall.
Gently press down crusty top to make evenly thick cake. Using small knife, cut around pan sides to loosen cake. Remove pan sides. Invert cake onto cooling rack. Peel off parchment paper.
glaze by melting chocolate with cream gently in microwave, stirring at
frequent intervals till smooth. Add corn syrup and stir to combine.
Place cake on rack set over baking sheet. Spread ½ cup glaze smoothly over top and sides of cake. Freeze for about 3 minutes to set glaze. Pour remaining glaze over cake; smooth sides and top. Place cake on platter. Chill until glaze is firm, about 1 hour. Serve at room temperature.