Add the butter, both types of chocolate chips, and vanilla to an extra large mixing bowl. Set aside.

Line a 9x13'' pan with parchment paper and set aside.

In a very large heavy-bottom saucepan combine the evaporated milk, sugar, and marshmallows. Cook over medium heat, stirring frequently until the mixture comes to a boil.

Boil for about 10 minutes* stirring constantly, being cautious to scrape down the sides of the pan and along the bottom.

Pour the hot mixture into the bowl with the chocolate chips and use an electric mixer to gently combine everything until melted and smooth. Pour fudge into prepared pan.

Chill completely in the refrigerator. Once cooled you can pull up on the edges of the parchment to lift the entire slab of fudge out of the pan.

Cut into small squares, or cut into a few "bricks," wrap in plastic wrap and then aluminum foil. Store fudge wrapped tightly in the fridge.