Prepare the Dish: Begin by lining an 8-inch-by-8-inch baking dish with parchment paper. This will make it easier to remove the bars once they are set.
Melt Butter & Sugar: In a medium saucepan, combine the 1 cup butter, ½ cup brown sugar, and 1 teaspoon pure vanilla extract. Heat over low until the butter is melted and the mixture is combined, stirring constantly to avoid burning.
Mix Oats & Spices: Add the 3 cups rolled oats, ½ teaspoon ground cinnamon, and ¼ teaspoon kosher salt to the melted mixture. Cook for 4-5 minutes, stirring constantly to ensure the oats are well coated and slightly toasted.
Layer Base: Pour half of the oatmeal mixture into the prepared baking dish. Use a spatula to press it down firmly, creating an even layer at the bottom of the dish.
Chocolate Layer: In a microwave-safe bowl, combine the 1 cup dark chocolate chips and ¾ cup chunky peanut butter. Microwave in increments of 30 seconds, stirring until smooth. Pour ⅘ of this melted chocolate-peanut butter mixture over the oats, reserving the rest for later.
Top with Oats: Spread the remaining oat mixture evenly over the chocolate layer, pressing down gently. Drizzle the reserved chocolate on top for a decorative finish.
Chill & Serve: Place the dish in the refrigerator for at least 4 hours, or until the bars are firm. Once set, remove from the dish using the parchment paper and cut into squares to serve.