Line a 9 inch square pan with foil and coat the foil with cooking spray.
Combine milk, brown sugar, butter and salt in a medium sized saucepan and bring to a boil, stirring until sugar is dissolved. Reduce heat to medium-low and simmer, stirring frequently for 8-10 minutes until mixture is 238°F on a candy thermometer or a teaspoon of the mixture dropped into a small bowl of cold water holds a soft ball when pressed between your fingers.
Transfer the mixture to a heatproof bowl. Beat in the vanilla with an electric mixer at medium speed, then add powdered sugar 1/4 cup at a time, beating until fudge is thick and smooth, about 5 minutes.
Quickly stir in 1 cup of chocolate chips and pour the fudge into the prepared pan, smoothing the top with a spatula. Press remaining 1/2 cup of chocolate chips into the top of the fudge.
Place the fudge in the refrigerator and chill for 1 hour or until the fudge is firm enough to cut.