Preheat oven to 350 degrees. Grease a 9 by 13 baking pan. In a large bowl, cream butter and sugars until smooth. Beat in eggs one at a time then stir in vanilla. Combine flour, baking powder and salt. Stir into batter just until blended. Fold in shredded yams. Spread into prepared pan. Bake for 30 minutes until a toothpick comes out clean. Mix together powdered sugar, butter and milk until smooth. Spread over brownies while still warm. Kids will never know they were yams!