Line an 8" square baking dish with parchment paper, coat with cooking spray then set aside.
Pour the entire can of sweetened condensed milk into a large microwave-safe bowl then add the butter and salt. Microwave on high for 1 full minute, just long enough to heat the milk and melt the butter.
Stir in vanilla and the entire bag of chocolate wafers—we used the minis because they melt so much faster, but it may need an extra 10 seconds in the microwave to get them all the way melted. Once the chocolate has melted completely, immediately fold in the marshmallows and walnuts then spread into prepared dish.
Let fudge cool to room temperature until firmly set, then slice and serve. Enjoy!
If you have all of your ingredients lined up and ready to go, this will be the quickest candy you will ever make!