Step 1

Line an 8 x 8 - inch with foil or parchment paper, and lightly grease with cooking spray. Set aside.

Step 2

Place the butter, chocolate chips, and sweetened condensed milk in a large, microwave-safe bowl. Heat it for 3 to 4 minutes (high power-see tips), stirring at 30 second intervals, until melted and smooth.

Step 3

Add the walnuts, and stir until incorporated.

Step 4

Transfer the fudge into the prepared pan, and smooth into an even layer. Refrigerate for 2 to 3 hours until set. Cut into squares and serve.

Step 5

Store leftovers in the refrigerator or at room temperature for up to 4 days.