Add the butter, both types of chocolate chips, and vanilla to an extra large mixing bowl. Set aside.
Line a 9×13'' 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. Keep the heat on medium and stir constantly, being cautious to constantly scrape down the sides of the pan and along the bottom of the pan as you stir (this will help the sugar dissolve and not be grainy, and keep anything from burning to the bottom or sides of the pan.)
Boil for about 10 minutes*. The fudge should be light golden brown in color. (If using a candy thermometer you will want to cook it to around 235-240 degrees F. Temperature will vary depending on altitude, humidity etc., but watch the clock and make sure it boils for about 10 minutes).
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 I like to cut it into a few "bricks", wrap in plastic warp and then aluminum foil. Store fudge wrapped tightly in the fridge so it doesn't dry out.
Video
Notes
Chocolate Chips: Use high quality brand, like Guittard or Ghirardelli, for best flavor and melting quality.Make Ahead and Freezing Instructions: Prepare chocolate fudge as instructed and, once cooled, wrap bricks of fudge in plastic wrap, and then aluminum foil. Store in the fridge for a few weeks, or in the freezer for 3-6 months.Variations:
Peanut Butter Fudge: Once chocolate is melted, stir in a few tablespoons of peanut butter until smooth.
Peppermint Fudge: Replace the vanilla with a teaspoon of peppermint extract.
Rocky Road Fudge: stir in 1 cup mini-marshmallows and a handful of your favorite nuts, at the end, just before adding to the pan.
Marbled Fudge: Use half white chocolate chips and divide the butter and vanilla evenly between two separate bowls (one with white chocolate and one with regular chocolate chips. Divide the hot mixture from the sauce between the two bowls, mixing bowl until the chips are melted and smooth. Add the regular chocolate to prepared pan, and pour the white chocolate mixture over the top. Use a bitter knife to swirl them together.
Add Nuts: stir in 1 cup walnuts, pecans, or your favorite kind of nuts, at the end, just before adding to the pan.