Preheat oven to 350. Line a 13 x 9 inch baking pan with foil, extending foil over edges of pan. Lightly coat foil with cooking spray. Set aside
For crust, in medium bowl, stir together flour, powdered sugar and salt. using a pastry blender, cut in the 3/4 cup butter til mixture resembles coarse crumbs. Press mixture evenly onto bottom of prepared pan. Bake for about 20 minutes or til lightly browned.
Meanwhile, for filling in medium bowl, mix eggs, mixed nuts, brown sugar, maple syrup, the 3 Tbsp. melted butter and maple flavoring. Spread filling evenly over hot crust.
Bake for about 20 minutes or til filling is set. Cool in pan on wire rack. using edges of foil, lift uncut bars out of pan. Cut into bars.
For garnish, if desired, in large glass measuring cup, mix white baking pieces and shortening. Microwave on full power for 1 minute or til mixture is melted. Stir every 30 seconds. Spoon or drizzle bars with white chocolate mixture and top each with a pecan half. Makes 36 bars.