Heat oven to 350º. In large bowl, beat sugar, ½ cup brown sugar and 1 cup margarine until light and fluffy. Add pumpkin, vanilla and egg; blend well. Add flour, baking powder, baking soda, cinnamon and salt; mix well. Stir in walnuts. Drop dough by rounded teaspoonfuls 2 inches apart onto ungreased cookie sheets.
Bake at 350º for 10-12 minutes or until light golden brown around edges. Immediately remove from cookie sheets. Cool completely.
In medium saucepan, combine 3 tbsp. margarine and ½ cup brown sugar. Bring to a boil. Cook over medium heat 1 minute or until slightly thickened, stirring constantly. Cool 10 minutes. Add milk; beat until smooth. Beat in enough powdered sugar for desired spreading consistency. Frost cooled cookies. Let stand until frosting is set.