In a large bowl, combine the flour, sugar, hazelnuts, baking powder and salt. Whisk the egg yolks, water, oil and vanilla; add to dry ingredients. Beat until well blended. In another large bowl, beat egg whites and cream of tartar on medium speed until soft peaks form; fold into batter.
Gently spoon into an ungreased 10-in. tube pan. Cut through the batter with a knife to remove air pockets. Bake on the lowest rack at 325° for 60-70 minutes or until top springs back when lightly touched. Immediately invert the pan onto a wire rack; cool completely. Carefully run a knife around edges and center tube to loosen; remove cake.
In a small bowl, cream butter and confectioners' sugar. Beat in the baking cocoa, vanilla and enough coffee to achieve desired consistency. Frost cake.
Nutrition Facts: 1 slice equals 399 calories, 22 g fat (7 g saturated fat), 87 mg cholesterol, 190 mg sodium, 47 g carbohydrate, 1 g fiber, 6 g protein.