Heat oven to 350°F. In large bowl, beat brown sugar and butter with electric mixer on medium speed until blended. Beat in vanilla and egg. On low speed, beat in oats, flour, baking soda and salt until well blended. Reserve 3/4 cup mixture for topping. Press remaining mixture in bottom of ungreased 13x9-inch pan.
In 2-quart saucepan, heat chocolate chips and condensed milk over medium heat about 5 minutes, stirring frequently, until chips are melted and mixture is smooth. Pour over crust.
Sprinkle chopped candy bars evenly over chocolate mixture. Crumble reserved oat mixture over candy.
Bake 32 to 37 minutes or until golden brown and set. Cool on cooling rack at least 2 hours. For bars, cut into 6 rows by 6 rows.