Line a 9×9-inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a large mixing bowl, beat the softened unsalted butter, light brown sugar, and granulated sugar together until creamy and smooth, about 2-3 minutes.
Add the vanilla extract and mix until combined.
Gradually add the all-purpose flour and salt to the butter mixture, mixing on low speed until fully incorporated and a dough forms.
Stir in 1 cup semi-sweet chocolate chips by hand until evenly distributed throughout the dough.
Press the cookie dough evenly into the prepared baking pan, smoothing the top with a spatula.
In a microwave-safe bowl, combine the 1 1/2 cups semi-sweet chocolate chips and 1 tablespoon vegetable oil. Microwave in 30-second intervals, stirring after each, until the chocolate is completely melted and smooth.
Pour the melted chocolate over the cookie dough layer and spread evenly with a spatula.
Refrigerate the bars for at least 2 hours or until the chocolate topping is firm.
Once set, lift the bars out of the pan using the parchment paper overhang and cut into 16 squares.
Store the bars in an airtight container in the refrigerator.