For thicker fudge : line a 9x9 baking dish with parchment paper or tin foil. Spray lightly with cooking spray. For thinner fudge : line a 9x13 baking dish with parchment paper or tin foil. Again, make sure to spray the tin foil or paper to avoid sticking.
In a large mixing bowl add white chocolate chips and marshmallow fluff. Set aside.
In a saucepan, over medium heat, add butter, sugar, heavy cream and salt. Bring to a boil and cook for 5 minutes, stirring occasionally so the sugar does not burn.
Pour butter mixture into the bowl with the white chips and marshmallow fluff. Let sit for 2 minutes so it can melt the chocolate and fluff.
Add the mint extract and use a handheld blender to mix everything together until combined and smooth.
Stir in crushed Oreos (make sure to reserve about ½ cup to sprinkle on top) and pour into prepared baking dish. Sprinkle the reserved Oreos on top and gently press down.
Refrigerate for 6-8 hours. Cut into small squares and trim edges if desired. I think the fudge gets better and less sweet after it's been in the fridge for 1 day.