Prep Chocolate Chips: 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.

Heat Ingredients: 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 degrees F. Reduce candy temp 1 degree for every 1,000ft of elevation as temperature will vary depending on altitude).

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.

Peanut Butter Swirl: Combine peanut butter, powdered sugar, and melted butter until smooth. Pour over hot fudge and use a butter knife to draw lines down and back the pan, and side to side, to swirl/marble the peanut butter throughout.

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 wrap and then aluminum foil. Store fudge wrapped tightly in the fridge so it doesn't dry out.