Melting butter in a large saucepan on medium heat. Add the brown sugar and vanilla, stirring. Mix in rolled oats. Simmer for 2 to 3 minutes or until ingredients are well blended. Put half of the batter in the bottom of the prepared pan. Save the other half for decoration.
Meanwhile, melt chocolate chips and peanut butter in a small saucepan over low heat, stirring constantly, until smooth. Pour the chocolate mixture over the crust in the pan, spreading it evenly with a knife or the back of a spoon.
Roll the remaining oatmeal mixture over the chocolate layer, pressing gently. Cover and refrigerate 2 to 3 hours or overnight. Bring it to room temperature before cutting it into bars.