Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a large mixing bowl, beat the softened 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 the mini semisweet chocolate chips evenly throughout the dough.
Press the cookie dough evenly into the prepared baking pan, creating a smooth and compact layer.
Pour the sweetened condensed milk over the cookie dough layer and spread it evenly with a spatula.
In a microwave-safe bowl, melt the 1 1/2 cups semisweet chocolate chips in 30-second intervals, stirring between each, until smooth.
Pour the melted chocolate over the sweetened condensed milk layer and spread evenly to cover the entire surface.
Refrigerate the bars for at least 2 hours or until the chocolate is fully set and firm.
Once set, use the parchment paper overhang to lift the bars from the pan and place on a cutting board.
Cut into 16 equal bars and serve chilled.