Set a rack in the middle level of the oven and preheat to 350 degrees. Butter a 12-cup Bundt pan and flour the buttered surface, shaking out the excess.
Place hazelnuts in the bowl of a food processor fitted with the steel blade and pulse continuously until finely ground.
Bring a small pan of water to a boil and remove from heat. Place chocolate in a heatproof bowl and set over the pan of water, stirring occasionally, until melted. Remove bowl from pan and set aside.
In a large mixer bowl beat the butter and 1/2 cup of the sugar until soft and light. Beat in the chocolate and scrape bowl and beater(s). Beat in the egg yolks, one at a time. Scrape bowl and beater(s). Stir in the ground hazelnuts and the flour by hand.
In a clean, dry mixer bowl, whip the egg whites with the salt on medium speed, until they are frothy. Continue whipping the egg whites until they are white and opaque, and beginning to hold their shape when the beater(s) are lifted. Increase the speed and gradually whip in the remaining 1/2 cup sugar, continuing to whip the egg whites until they hold a soft, glossy peak.
Stir a quarter of the egg whites into the batter to lighten it, then fold in the remaining egg whites with a rubber spatula, continuing to fold until no streaks of egg white remain. Scrape the batter into the prepared pan.
Bake the cake about 35 to 45 minutes, until it is well risen and a toothpick or skewer inserted between the edge of the pan and the central tube emerges clean. Place a rack on the pan and invert cake and pan to rack to cool. Remove pan after 5 minutes and allow cake to cool.
For glaze, combine water, corn syrup and sugar in a saucepan and stir well to mix. Place over low heat and bring to a boil, stirring occasionally to dissolve sugar. Remove from heat and add chocolate. Swirl pan so that chocolate is submerged in hot syrup, then allow to stand 2 minutes. Whisk glaze smooth.
Before glazing cake, cut a disk that will fit under the cake from stiff corrugated cardboard. Cut a hole in the canter corresponding to the central tube in the cake pan. Slide the cardboard under the cake and place it on a rack over a jelly roll pan. Pour glaze from pan onto highest point of cake, all around, allowing glaze to drip down outside and center of cake. Let glaze dry before moving cake.
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