HEAT oven to 350 degrees F. Spray 13 x 9-inch pan with a no-stick cooking spray. Melt chocolate chips in dry, microwave-safe bowl on HIGH (100% power) 1 minute. Stir. Microwave at additional, 10- to 15-second intervals, stirring just until chips are melted. Set aside.
BEAT together brown sugar, peanut butter and butter in large bowl with an electric mixer, until smooth and creamy. Add egg and vanilla. Beat until well blended.
STIR together flour, baking soda and salt. Add to peanut butter mixture. Beat just until combined. Stir in oats. Press 3/4 of dough into prepared baking pan. Spread evenly with melted chocolate. Dot chocolate layer with remaining dough.
BAKE 24 to 26 minutes. Cool. Cut into 24 bars.