Spoon batter evenly into 3 greased and floured 9-inch square pans. Bake at 350° for 25 to 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire rack 10 minutes. Remove from pans, and let cool completely on wire rack.
Spread Bourbon Buttercream Frosting between layers and on top and sides of cake, sprinkling 1/4 cup pecans on top of frosting for each of the layers. Press remaining 2 cups pecans into sides of cake. Sprinkle chopped semisweet chocolate bar over top of cake.