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