Line an 8x8 glass dish with foil and lightly spray with nonstick cooking spray. Set aside.
Optional: Chop your walnuts and toast them in a pan for a couple of minutes.
In a large microwave-safe bowl, microwave chocolate chips and sweetened condensed milk in 30-second increments until smooth and completely melted. Stir in walnuts. Pour into prepared pan and smooth the top.
Bring to room temperature before refrigerating. Once it's pretty solid, remove it from the pan using the foil and cut it into squares. Serve.