Heat oven to 350Â°F. Grease and flour two 9-inch round cake pans. Sprinkle pecans evenly in bottom of greased and floured pans.
Lightly spoon flour into measuring cup, level off. In large bowl, combine all remaining cake ingredients except egg whites and chocolate; beat 1 1/2 minutes at medium speed. Add egg whites; beat 1 1/2 minutes. Carefully spoon 1/4 of batter into each pecan-lined pan. Sprinkle with grated chocolate. Spoon remaining batter over grated chocolate; spread carefully.
Bake 350Â°F. for 30 to 40 minutes or until cake is golden brown and top springs back when touched lightly in center. Cool 10 minutes; remove from pans. Cool 1 hour or until completely cooled.
Meanwhile, in small saucepan, combine 1/2 cup sugar, unsweetened chocolate and 1/4 cup water, cook over low heat until melted, stirring constantly until smooth. Remove from heat; cool.
In small bowl, combine 1/2 cup shortening and 1 teaspoon vanilla. Gradually beat in 2 cups of the powdered sugar until well blended. Reserve 1/3 cup white frosting. To remaining frosting, add cooled chocolate, remaining 1/4 cup powdered sugar and enough water for desired spreading consistency.
To assemble cake, place 1 layer, pecan side up, on serving plate. Spread with about 1/2 cup chocolate frosting. Top with second layer, pecan side up. Frost sides and 1/2 inch around top edge of cake with remaining chocolate frosting. If necessary, thin reserved white frosting with enough water for desired piping consistency; pipe around edge of nuts on top of cake.