Line an 8x8-inch with parchment paper, leaving an overhang to lift the fudge out later. Cut the butter into tablespoon pieces. Coarsely chop enough Oreos to get 2 cups. Crush the remaining Oreos in a bag with a rolling pin or meat mallet for the topping and set aside.
Add the butter, chocolate chips, and sweetened condensed milk to a medium nonstick pan. Warm over the lowest heat and stir constantly with a heat-resistant spatula, scraping the sides and bottom often. Keep the heat low so the chocolate stays smooth.
Once the butter and chocolate are fully melted, remove from heat. Stir in the salt and vanilla. Add the 2 cups chopped Oreos and gently mix to combine.
Spread the mixture into the lined pan. Spray a spatula with cooking spray and smooth into an even layer. Sprinkle the crushed Oreos on top and lightly press them in. Cover and refrigerate for 4–8 hours, or until firm.
Lift the fudge out using the parchment overhang. Cut with a hot, sharp knife (run it under hot water, wipe dry, and make quick cuts). The fudge is rich, so small pieces are best.
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Notes
Note 1: Milk chocolate makes this fudge extra rich and sweet. For a less sweet fudge, use semi-sweet or dark chocolate, or do half milk chocolate and half semi-sweet. High-quality chocolate chips (like Ghirardelli or Guittard) give the best flavor.Storage: The fudge tastes best in the first 1–2 days before it begins to dry out. If you want it to last longer, leave it as a full block, wrap it tightly, seal it in a bag, and store in the fridge or freezer. After slicing, wrap individual pieces and keep them at room temp.