In a microwave safe bowl; combine dark chocolate chips with sweetened condensed milk.
Line a square 9x9 pan with foil or parchment paper and generously grease with butter or shortening. Spread crushed Oreos evenly on bottom of pan.
Microwave chocolate 30 seconds, stir. Microwave 15 seconds then add salt and vanilla. Stir vigorously until chocolate is smooth. Microwave 15 seconds more if necessary.
Transfer mixture into prepared pan and spread into an even layer.
Refrigerate at least 4 hours to set.
Remove fudge and foil from pan. Carefully peel all foil from fudge.