Wrap these chewy treats in plastic wrap, and place them in a zip-top plastic bag; freeze up to three months, if desired.
Combine cake mix and brown sugar in a mixing bowl. Add eggs and melted butter, beating at medium speed with an electric mixer until blended (batter will be stiff). Spoon half of batter into a lightly greased 13- x 9-inch pan; sprinkle with 1 cup each of dates and walnuts.
Stir remaining 1 cup each of dates and walnuts into remaining batter; spread over mixture in pan.
Bake at 350° for 30 to 35 minutes or until golden. Run a knife around edge of pan to loosen sides. Let stand 30 minutes before cutting. Cut into squares, and store bars in an airtight container.