Line a 9 by 9 inch pan with aluminum foil.
Over medium low heat, add the sweetened condensed milk to a small saucepan. Whisk in the cocoa powder.
Stir in the chocolate chips and stir consistently until well combined. Stir in the vanilla extract.
Pour into the pan, and add the marshmallow bits, gently pressing them into the fudge. (You may not need all of them.)
Refrigerate for at least four hours, covered, before removing the fudge from the pan by lifting the aluminum foil, peeling it off the fudge, and cutting the fudge to serve.
Store in an airtight container at room temperature for up to two weeks.