Line 8x8 glass dish with foil and spray with nonstick cooking spray lightly. Set aside.
In a large microwave-safe bowl, stir chocolate chips and sweetened condensed milk together and microwave in 30 second increments until smooth and completely melted. Stir in chopped walnuts. Pour into prepared pan and smooth the top.
Bring to room temperature {about 1 hour} before refrigerating. Once it's pretty solid, remove from the pan using the foil and cut into squares. Serve.