Line a 9-inch square baking dish with parchment paper; set aside; have the paper hang over the edges to easily remove bars at the end
Melt the butter and brown sugar in a saucepan over medium heat until the butter has melted and the sugar has dissolved*; stir in vanilla; mix in the oats
Cook over low heat for 3-4 minutes until ingredients are well blended
Pour half of the oat mixture into the prepared baking dish; spread out the mixture evenly and press down; reserve the other half for the second layer
Filling: melt the peanut butter and chocolate chips in a microwave safe bowl; stir until smooth
Pour the chocolate mixture over the oat mixture in the pan; reserve 1/4 cup for drizzling later; spread evenly over the oat mixture
Pour the remaining oat mixture over the chocolate layer; press in gently and drizzle with the remaining chocolate mixture
Refrigerate 2-3 hours; bring to room temperature before cutting into squares
*Make sure not boil the sugar mixture too long or the end result will be dry and crumbly bars
