The black walnut filling adds distinct flavor and color to these cookies. Black walnuts, although tough nuts to crack, have flavor worth the effort of cracking them and picking out the nutmeats.
In a large mixing bowl beat shortening with an electric mixer on medium to high speed for 30 seconds. Add the 1/2 cup brown sugar, baking soda, and salt; beat until combined,scraping sides of bowl occasionally. Beat in egg until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour. Cover and chill dough for 1 to 2 hours or until firm enough to handle.
Meanwhile, combine melted butter, the 2/3 cup brown sugar, walnut flavoring (if desired), and walnuts.
On a lightly floured surface, roll dough into a 12-inch square. Spread walnut mixture evenly over dough; roll up jelly-roll style; seal edges. Wrap in plastic wrap. Chill in the refrigerator at least 3 hours.
Using a sharp knife, cut dough into 1/4-inch-thick slices. Place slices 2 inches apart on an ungreased cookie sheet.
Bake in a 375 degree F oven for 7 to 9 minutes or until edges are lightly browned. Cool on baking sheet for 1 minute. Transfer cookies to a wire rack; cool. Makes about 48 cookies.