Heat oven to 350°F. Grease 8-inch round cake pan. In medium saucepan, melt butter and 16 oz. chocolate over medium-low heat, stirring until smooth. Remove from heat. Stir in 2 teaspoons vanilla. Gently stir in eggs until well combined. Pour into greased pan. Place cake pan in 13x9-inch pan; add warm water until 1 inch deep.
Bake at 350°F. for 35 to 40 minutes or until center is set. Remove cake pan from water bath; place on wire rack. Cool 40 minutes.
Carefully run knife around edge of pan; invert cake onto wire rack. Cool an additional 20 minutes.
Meanwhile, in small saucepan, combine whipping cream, corn syrup and 1 teaspoon vanilla. Bring to a boil over medium heat, stirring occasionally. Remove from heat. Add 3 oz. chocolate; stir until melted and smooth.
Place cake on serving platter. Place pieces of waxed paper under cake to catch drips. Slowly pour glaze over top and sides of cake to cover. With narrow metal spatula, smooth glaze over cake. Sprinkle hazelnuts around top edge of cake. When glaze is set, remove waxed paper. Store in refrigerator.