Lightly grease a 9-inch square baking pan and set it aside.

In a large saucepan over medium heat, melt the butter. Stir in the brown sugar and vanilla, then mix in the oats. Let everything cook together over low heat for about 2 to 3 minutes until well combined.

Press half of the oat mixture firmly into the bottom of the prepared pan to form the crust. Set the rest aside for later.

In a smaller saucepan, gently melt the chocolate chips and peanut butter together over low heat, stirring often until smooth and glossy.

Pour the chocolate-peanut butter mixture over the oat crust and spread evenly with a spatula or the back of a spoon.

Crumble the remaining oat mixture over the top layer, pressing it down gently so it sticks.

Cover and refrigerate for 2 to 3 hours (or overnight, if you’ve got the patience!). Let it come to room temperature before cutting into bars.