1Heat oven to 350°F. Grease cookie sheets with shortening or cooking spray. In large bowl, beat granulated sugar, 1/2 cup butter, the buttermilk, vanilla, egg and 2 oz melted chocolate with electric mixer on medium speed until well blended. Stir in flour, baking soda and salt until dough forms. Stir in nuts.
2Onto greased cookie sheets, drop dough by level 1/4 cupfuls or #16 cookie/ice cream scoop about 2 inches apart. Flatten slightly with fork.
3Bake 13 to 15 minutes or until almost no indentation remains when touched in center. Cool 1 to 2 minutes; remove from cookie sheets to cooling racks. Cool completely, about 15 minutes.
4In 2-quart saucepan, melt 2 oz chocolate and 2 tablespoons butter over low heat, stirring frequently. Remove from heat. Stir in powdered sugar and hot water until smooth. (If frosting is too thick, stir in additional water. If frosting is too thin, stir in additional powdered sugar.) Spread frosting on cooled cookies. If desired, draw tines of fork through frosting to make wavy lines. Decorate with candy sprinkles.