Melt butter in a large saucepan over medium heat. Stir in brown sugar and vanilla. Mix in the oats. Cook over low heat 2–3 minutes or until ingredients are well-blended.
Meanwhile, melt chocolate chips and peanut butter in a small heavy saucepan over low heat, stirring frequently until smooth.
Press half of the mixture into the bottom of the prepared pan. Reserve the other half for topping. Pour the chocolate mixture over the crust in the pan, and spread evenly with a knife or the back of a spoon. Crumble the remaining oat mixture over the chocolate layer, pressing in gently.
Cover and refrigerate 3 hours or overnight.
Bring to room temperature before cutting into bars.