Line an 8-inch or 9-inch square baking dish with parchment paper, ensuring the edges hang over the sides for easy removal.

In a large saucepan, combine ½ cup of butter and 1 cup of brown sugar over medium heat, stirring until melted and dissolved.

Add 1 teaspoon of pure vanilla extract to the mixture and mix well.

Mix in 3 cups of rolled oats into the melted mixture, reduce heat to low and cook for an additional 3-4 minutes, stirring continuously.

Spread half of the oat mixture evenly across the bottom of the prepared dish and press it down firmly.

In a separate bowl, combine 1 cup of chocolate chips and ½ cup of peanut butter, melting until smooth.

Pour the chocolate-peanut butter mixture over the pressed oat layer, reserving ¼ cup for drizzling later.

Take the remaining oat mixture and spread it over the chocolate filling, then drizzle reserved chocolate over the top.

Chill in the refrigerator for 2-3 hours until firm, then remove and allow to come to room temperature.

Lift the bars from the dish using parchment paper, slice into squares or bars, and enjoy.