Open a small hole in the bag of potato chips and gently crush them. Pour the crushed chips into a large mixing bowl and set aside.
Line a cookie sheet or 9×13 pan with parchment paper and set aside.
In a saucepan over medium heat, combine the peanut butter, corn syrup, and sugar. Stir frequently until the mixture is smooth and well blended, about 8–10 minutes. Do not overcook — cooking longer can cause the candy to become too hard.
Remove the saucepan from the heat and stir in the vanilla.
Pour the hot peanut butter mixture over the crushed potato chips. Stir gently until the chips are evenly coated.
Spread the mixture evenly onto the prepared pan, pressing lightly to level the surface.
In a microwave-safe bowl, combine the chocolate chips and butter (and milk, if using). Microwave in 10-second intervals, stirring well after each interval, until smooth and fully melted.
Allow the bars to cool completely at room temperature until set. Once firm, cut into bars and serve.