Preheat oven to 350 degrees F (175 degrees C). Butter and flour a 12-cup Bundt pan.

Grind hazelnuts in a food processor until finely ground.

Melt chocolate in a heatproof bowl over simmering water, stirring occasionally.

In a large mixer bowl, beat butter and 1/2 cup sugar until light and fluffy.

Beat in the melted chocolate, then egg yolks one at a time. Stir in ground hazelnuts and flour.

In a separate bowl, whip egg whites with salt until frothy.

Gradually add the remaining 1/2 cup sugar and whip until soft, glossy peaks form.

Fold a quarter of the egg whites into the batter to lighten it, then gently fold in the remaining egg whites until just combined.

Pour the batter into the prepared Bundt pan.

Bake for 35-45 minutes, or until a toothpick inserted into the cake comes out clean.

Invert the cake onto a rack to cool completely before removing the pan.

For the glaze, combine water, corn syrup, and sugar in a saucepan. Bring to a boil, stirring until sugar dissolves.

Remove from heat and add chocolate. Let stand for 2 minutes, then whisk until smooth.

Place cake on a cardboard disk on a wire rack over a jelly roll pan.

Pour the glaze over the cake, allowing it to drip down the sides and center.

Let the glaze dry completely before moving the cake.