Preheat oven to 375ºF. Combine flour, baking soda, and salt in a small bowl. Set aside. Coarsely chop 5 of the chocolate squares; set aside. Beat margarine, brown sugar, and Splenda (or sugar) in large bowl with electric mixer on medium speed until light and fluffy. Add egg and vanilla; mix well. Gradually add flour mixture, beating until blended. Stir in chopped chocolate and walnuts. Drop heaping tablespoons of cookie dough, 2 inches apart, onto ungreased baking sheets. Bake cookies 11 to 12 minutes or until lightly browned. Cool one minute; remove from baking sheets. Cool completely on wire racks. Melt remaining 3 chocolate squares in microwave as directed on package. Dip ½ of each cookie into melted chocolate. Place in a single layer on wax paper-covered plates. Let stand until chocolate is set. Yield: 3 dozen cookies.