Preheat oven to 350°. Butter a 12-cup Bundt or tube pan. Coat with 1/2 cup ground hazelnuts.
Beat butter and 6 tablespoons of sugar on medium speed of a heavy-duty mixer. Mix until soft and light, about 5 minutes. Add egg yolks one at a time, beating after each addition.
In a separate bowl, stir together flour, baking powder, ground hazelnuts, and chocolate. Add half of the flour mixture to the egg yolk mixture. Mix on low speed just until blended. Beat in the milk. Beat in the remaining flour mixture.
Beat egg whites on medium speed with a mixer’s whisk attachment. Mix until white and opaque and just beginning to hold a shape. Increase speed to medium-high and add remaining 6 tablespoons sugar in a stream. Continue mixing until soft peaks form.
Stir one-third of egg whites into batter. Then, fold in remaining egg whites.
Pour batter into prepared pan. Bake for 50-60 minutes, or until a toothpick inserted in the middle comes out clean.
Cool in pan on wire rack for 5 minutes. Then, remove cake from pan and cool completely on rack.