Heat oven to 350°F. Grease 13x9-inch pan. In medium bowl, combine flour, baking powder and salt; mix well.
Melt 1/2 cup butter in large saucepan over low heat. Remove from heat. Add flour mixture, brown sugar, cashews, 1 teaspoon vanilla and eggs; mix well. Spread batter in greased pan.
Bake at 350°F. for 25 to 30 minutes or until golden brown and bars begin to pull away from sides of pan. Cool 30 minutes or until completely cooled.
Melt 2/3 cup butter in medium saucepan over medium heat. Cook 13 to 15 minutes or until butter is light golden brown, stirring constantly. Remove from heat. Stir in powdered sugar, 1/2 teaspoon vanilla and enough milk for desired spreading consistency. Immediately spread icing over cooled bars. Cut into bars.