add butter and brown sugar to a medium-sized pot, and melt over medium heat until the brown sugar has all dissolved.
allow to simmer on a low heat for 3-5 min.
add vanilla and stir
add oats and fold in until everything is well combined
line an 8x8 baking pan with parchment paper
add half the oat mixture to the pan, and spread evenly, do not pack down tightly.
add chocolate chips and peanut butter to a microwave-safe bowl and place in the microwave for 30 seconds at a time, stirring in between until everything is melted and smooth.
pour the mixture over the oat base in the pan saving a small amount for drizzling on top.
add the remaining amount of oat mixture to the top, and drop evenly over the whole square trying not to move too much of the chocolate layer.
do not pack down, only lightly spread to fill in all gaps
using a spoon drizzle remaining amount of chocolate mixture over the top to add a little pizzaz
cover and refrigerate for 2-3 hours or over night.
allow the squares to sit for 10 min or so on the counter before slicing as the bars can be quite hard when cold.
slice with a sharp knife into 9 squares (3x3) or 16 (4x4) depending on the desired size. store in an airtight container in the fridge for up to 1 week.