1.Preheat oven to 180°C. Grease a deep 20cm square loose-based cake pan.
2.Combine peanut butter, butter, brown sugar and sifted icing sugar in a medium bowl; press mixture evenly over base of pan. Bake 10 minutes.
3.Meanwhile, stir chocolate and nuts in small saucepan over low heat until chocolate is melted. Pour over base. Refrigerate 3 hours or until set before cutting.