Heat oven to 350º F. Coat a 13x9-inch pan with cooking spray. Set aside.
Beat brown sugar and butter in large bowl with electric mixer at medium high speed until light and fluffy. Beat in vanilla and egg.
Use a large rubber spatula to mix in the muffin mix and oats until well blended. Reserve one cup of mixture. Set aside. Pat remaining mixture into prepared pan with oiled hands or spray hands with cooking spray.
Combine condensed milk and chocolate chips in a medium saucepan. Cook over low heat until chocolate is melted, stirring constantly. Remove from heat. Stir in walnuts.
Quickly spread chocolate mixture evenly over muffin mixture in pan covering completely to edge of pan. Drop reserved oat mixture by teaspoonfuls over chocolate mixture. Spread slightly.
Bake for 35-40 minutes or until golden brown. Cool completely on wire rack. Cut into bars.