In 2-quart saucepan, heat 1/2 cup butter over medium heat, stirring constantly, until golden brown; remove from heat. Using spoon, gradually beat in powdered sugar. Stir in vanilla and the milk, 1 tablespoon at a time, until frosting is smooth and desired spreading consistency. Immediately spread over cooled bars. Score into 6 rows by 4 rows, making shallow cuts in frosting. Top center of each bar with cashew half, pressing in lightly. Cut bars on scored lines.