Heat oven to 375°F. Coat 15 x 10 x 1-inch jelly-roll pan with cooking spray.
Combine flour and salt in a small bowl.
Beat butter, shortening and brown sugar in large bowl until smooth and creamy. Beat in egg and vanilla. Stir in flour mixture and pecans. Spread in prepared pan.
Bake in 375°F oven 15 to 20 minutes, until lightly browned and wooden pick inserted in center comes out clean. Cool pan on rack.
Meanwhile, heat the baking chips in the top of a double boiler over hot, not boiling, water, stirring until chips are melted and smooth.
Drizzle cookie with melted chips.
Cut into bars. Store in tightly sealed plastic container with sheets of waxed paper between layers. If not serving right away, do not cut or drizzle with melted chips. Cover tightly with heavy-duty foil; seal in heavy-duty plastic bag, or place pan inside a large plastic container and seal; freeze. To serve, bring to room temperature, then drizzle with melted white baking chips.