Line a 20 centimeter square baking pan with parchment paper.
In a medium saucepan, melt butter over medium heat. Stir in brown sugar, cinnamon and vanilla.
Stir in oats, reduce heat to low and cook, stirring constantly, for four minutes.
Press half the oat mixture into bottom of prepared pan; transfer remaining oat mixture to a small bowl.
In the same saucepan, over low heat, melt chocolate chips and peanut butter, stirring until smooth; reserve 1/4 cup (60 milliliters) for drizzling.
Spread remaining chocolate mixture evenly over crust in pan.
Sprinkle reserved oat mixture over chocolate layer and gently press it in.
Drizzle reserved chocolate mixture over top. Cover and refrigerate for three hours, then let come to room temperature for 15 minutes before cutting into squares.