1Heat oven to 400ºF. Place paper baking cup in each of 12 regular-size muffin cups, or grease bottoms only of muffin cups.
2In medium bowl, stir Muffin Mix, Walnuts, milk, oil, egg and chocolate chips just until blended. Divide batter among muffin cups (each about two-thirds full).
3In small bowl, stir together brown sugar and butter; sprinkle over batter in each cup.
4Bake 17 to 21 minutes or until golden brown and tops spring back when lightly touched. Cool 5 minutes (if you did not use paper baking cups, run knife around edges of cups before removing); remove from pan.