In a small mixing bowl, cream butter and sugar until light and fluffy. Beat in flour. Press into a greased 15-in. x 10-in. x 1-in. baking pan. Bake at 350° for 12-15 minutes or until golden brown.
Meanwhile, in a large saucepan, melt chocolate with corn syrup over low heat; stir until smooth. Remove from the heat. Stir in the sugar, eggs and vanilla. Add pecans.
Spread evenly over hot crust. Bake for 25-30 minutes or until firm around the edges. Cool on a wire rack. Melt chocolate and shortening; stir until smooth. Drizzle over bars. Yield: 4 dozen.