Lightly grease a small shallow square container with nonstick spray or butter and set aside.
Melt the butter and brown sugar in a saucepan over medium heat, until the butter has melted and the sugar has dissolved. Stir in the vanilla and mix in the oats.
Cook over low heat for 3 to 4 minutes, or until ingredients are well blended.
Pour half of the oat mixture into the prepared container. Spread out the mixture evenly, pressing down. Reserve the other half for the second layer.
To make the filling, melt the peanut butter and chocolate chips together in a small microwave-safe bowl and stir until it’s smooth. (heat for 20 second increments and stir until the chocolate is melted)
Pour the chocolate mixture over the top of the first layer, reserving about a 1/4 of it for drizzling at the end.
Pour the remaining oat mixture over the chocolate layer, pressing in gently and drizzle with the remaining chocolate mixture.
Refrigerate for 30 minutes or until cool before cutting into bars.