Grease a 9 inch square pan. Melt butter in large saucepan over medium heat. Add sugar and vanilla; mix well. Stir in oats. Cook over low heat 2 to 3 minutes or until ingredients are well blended. Press ½ of the mixture into greased pan. Use back of large spoon to spread mixture evenly. Meanwhile, melt chocolate chips in a heavy saucepan over low heat, stirring occasionally. Stir in peanut butter until smooth. Pour chocolate mixture over oat mixture in pan; spread evenly with knife or back of spoon. Crumble remaining oat mixture over chocolate layer, pressing in gently. Cover