Line an 8-inch or 9-inch square baking dish with parchment paper, leaving it hanging over the edges for easy removal later.

Melt 1/2 cup of butter and 1 cup of brown sugar over medium heat in a large saucepan, stirring continuously until fully melted and smooth (about 3-5 minutes). Remove from heat and stir in 1 teaspoon of vanilla extract.

Add 3 cups of rolled oats to the melted mixture, stirring well. Cook over low heat for an additional 3-4 minutes, stirring frequently until slightly toasted and fragrant.

Press half of the oat mixture firmly into the bottom of the prepared baking dish.

In a separate bowl, combine 1 cup of chocolate chips and 1/2 cup of peanut butter. Microwave in 30-second intervals, stirring until smooth and fully melted (about 1-2 minutes).

Pour the melted chocolate mixture evenly over the pressed oat layer, reserving about 1/4 cup for drizzling later.

Sprinkle the remaining oat mixture over the chocolate filling, pressing down gently to adhere.

Drizzle the reserved chocolate mixture over the top of the bars. Cover with plastic wrap and refrigerate for 2-3 hours or overnight until set.