For CRUST: Beat flour, butter, powdered sugar and salt together in a large bowl with mixer at low speed until mixture resembles coarse crumbs. Press firmly into a greased 15 x 10 x 1-inch baking pan. (Baking pan must be at least 1 inch deep; pan will be full.)
Bake 15 to 17 minutes or until edges of crust are golden. Prepare filling while crust is baking.
For FILLING: Beat eggs, corn syrup, brown sugar, flour, maple extract and salt in a large bowl until well blended. Stir in walnuts. Pour over hot crust; spread evenly.
Bake 15 to 20 minutes or until filling is set. Cool on wire rack; cut into bars.
