Line an 8×8-inch pan with foil or parchment for easier cleanup; set aside.
To a large, microwave-safe bowl, add the chocolate chips, sweetened condensed milk, butter, and heat on high power to melt, about 2 and a half minutes. Stop to check and stir after 2 minutes and heat from there in 30-second intervals until the mixture can be stirred smooth.
Add the walnuts and cranberries, and stir to combine.
Turn the mixture out into the prepared pan, smoothing the top lightly with a spatula as necessary. Place on the counter to cool and set up fully, about 4 hours. If you are in a hurry, you can place the pan in the fridge to expedite it.
Slice and serve after the fudge has set up.
