Sprinkle 2 cups pecans evenly into 3 generously buttered 9-inch round cakepans; shake to coat bottoms and sides of pans.
Beat 1/2 cup butter and shorteningat medium speed with an electric mixer until fluffy; gradually add sugar, beating well. Add egg yolks, 1 at a time, beating until blended after each addition. Stir in vanilla.
Add flour and baking soda to butter mixture alternately with buttermilk, beginning and ending with flour. Beat at low speed until blended after each addition. Stir in remaining 1 cup finely chopped pecans.
Beat egg whites at medium speed until stiff peaks form; fold one-third of egg whites into batter. Fold in remaining egg whites. (Do not overmix.) Pour batter into prepared pans.
Bake at 350° for 25 minutes or until done. Cool in pans on wire racks 10 minutes. Invert layers onto wax paper-lined wire racks. Brush tops andsides of layers with corn syrup, and cool completely.
Spread half of Pecan Pie Filling on 1 layer, pecan side up. Place second layer, pecan side up, on filling; spread with remaining filling. Top with remaining layer, pecan side up.
Arrange Pastry Garnish on and around cake, if desired.