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.
14 ounces sweetened condensed milk, 1 tablespoon unsweetened cocoa powder
Lower the heat a little, and stir in the chocolate chips. Stir consistently until well combined. You are looking for the chocolate chips to be just melted. Be careful not to overheat. Stir in the vanilla extract.
3 cups milk chocolate chips, 2 teaspoons vanilla extract
Pour into the pan, and add the marshmallow bits, gently pressing them into the fudge. (You may not need all of them.)
1 cup marshmallow bits
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.