In a large microwave-safe bowl, combine chocolate chips, condensed milk, and butter. Microwave on medium power until the chips are melted, about 3 to 5 minutes, stopping to stir once or twice during cooking.
While that's heating, generously grease an 8-inch square glass baking dish.
Once the chocolate mixture is smooth, stir in the walnuts. Pour into the prepared dish.
Refrigerate until the fudge sets for about 2 hours.
After set, cut into 16 squares and serve chilled.
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