Grease a 9×9-inch pan and set aside.
In a saucepan over medium heat, melt butter. Stir in brown sugar and vanilla until combined.
Remove from heat and mix in oats until evenly coated. Press half the oat mixture firmly into the bottom of the prepared pan.
In a small saucepan over low heat, gently melt chocolate chips with peanut butter, stirring until smooth. Pour over the oat crust and spread evenly.
Crumble the remaining oat mixture over the chocolate layer and press lightly to secure.
Cover and refrigerate 2–3 hours or overnight. Bring to room temperature briefly before cutting into bars.

For firmer bars, chill overnight.
Swap peanut butter for almond butter or sunflower seed butter for a nut-free option.
Stir chopped nuts or shredded coconut into the oat mixture for extra texture.
Press the top layer with the back of a spoon to create a neater finish.