HEAT oven to 350º F. Coat a 13 x 9-inch pan with no-stick cooking spray. Beat brown sugar and butter in large bowl with electric mixer at medium high speed until light and fluffy. Beat in vanilla and egg.
STIR in muffin mix. Stir in oats until well blended. Reserve one cup of mixture. Set aside. Pat remaining mixture into prepared pan with oiled hands.
COMBINE condensed milk and chocolate chips in medium saucepan. Cook over low heat until chocolate is melted, stirring constantly. Remove from heat. Stir in walnuts. Quickly spread evenly over muffin mixture covering completely to edge of pan. Drop reserved oat mixture by teaspoonfuls over chocolate mixture. Spread slightly.
BAKE 35 to 40 minutes or until golden brown. Cool completely on wire rack. Cut into bars.