Start by greasing a 9x13 inch baking dish with a little butter or lining it with parchment paper for easy removal later.
In a microwave-safe bowl, combine the unsalted butter and granulated sugar. Microwave in 30-second intervals, stirring in between, until the mixture is completely melted and smooth.
Once the butter and sugar mixture is melted, stir in the creamy peanut butter and milk. Mix until well combined and smooth.
Next, add the vanilla extract and salt to the mixture. Stir until everything is well incorporated.
In a large mixing bowl, combine the old-fashioned oats with 1 cup of the semisweet chocolate chips. Pour the peanut butter mixture over the oats and chocolate chips, stirring until everything is well coated.
Pour the mixture into the prepared baking dish and press it down firmly with a spatula to create an even layer.
Sprinkle the remaining 1 cup of chocolate chips on top of the pressed mixture, gently pressing them in so they stick.
Cover the dish with plastic wrap and refrigerate for at least 2 hours, or until the bars are firm.
Once set, remove the bars from the refrigerator. Lift them out of the dish using the parchment paper (if used), and cut into squares or rectangles.