Heat oven to 350°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 at 350°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.