Combine the chocolate chips, milk, and marshmallows in a medium-size saucepan over medium heat. Stir frequently as the chocolate melts. Add the peanut butter and vanilla. Stir to melt and combine well.
Remove from the heat. Lightly grease a 10-inch square pan or line with parchment. Pour the fudge into the pan and spread with a spatula. Chill in the refrigerator until firm. Slice into 1-inch pieces and store in an airtight container in the refrigerator. Enjoy!
This fudge can be made ahead of time and then kept in the freezer for several months. Thaw in the refrigerator when ready to serve.