Add the butter, both types of chocolate chips, and vanilla to an extra large mixing bowl. Set aside.
Line a 9×13'' pan with tin foil and set aside.
In a very large heavy 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 just watch the clock and make sure it boils for about 10 minutes).
Grease your foil-lined pan with cooking spray.
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 into prepared pan.
Chill completely in the refrigerator. Once cooled you can pull up on the edges of the tinfoil 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" that I wrap in tinfoil and give as gifts. Store fudge wrapped tightly in the fridge so it doesn't dry out.
*The hot boiling liquid can sometime splatter. To avoid burning yourself, wear kitchen gloves or an oven mit.To Store: Store fudge in an airtight container at room temperature for 2 weeks or in the refrigerator for about a month.To Make Ahead: Chocolate fudge is the ideal make ahead treat. the high amount of sugar and fat in this recipe help act as a preservative, allowing fudge to be made ahead and stored for several weeks.To Freeze: Allow fudge to cool completely and harden in the refrigerator. Then cut it into cubes (or leave it as a block) and wrap it tightly in plastic wrap and tinfoil. Freeze fudge for up to 3 months.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: At the end, stir in 1 cup mini-marshmallows and a handful of your favorite nuts.Marbled Fudge: Substitute white chocolate for some of the chocolate chipsAdd Nuts: Add the end, stir in 1 cup walnuts, pecans, or your favorite kind of nuts.