Heat oven to 350°F. In large bowl, combine sugar, 1/2 cup brown sugar and 1 cup margarine; beat until light and fluffy. Add pumpkin, vanilla and egg; blend well. Add flour, baking powder, baking soda, cinnamon and salt; mix well. Stir in nuts. Drop dough by rounded teaspoonfuls 2 inches apart onto ungreased cookie sheets.
Bake at 350°F. for 10 to 12 minutes or until light golden brown around edges. Immediately remove from cookie sheets. Cool 15 minutes or until completely cooled.
Meanwhile, in medium saucepan, combine 3 tablespoons margarine and 1/2 cup brown sugar; bring to a boil. Cook over medium heat for 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.