Have Ingredients Measured and Ready: Pull
out all ingredients, tools and equipment you’ll need before starting.
Prepare the Pan: Grease a 9x13 baking dish or line it with parchment paper for easier removal. (The larger the pan, the thicker the fudge, but the longer it will take to chill and set up.)
Combine Ingredients: In a medium saucepan over medium heat, add the sugar, butter, and evaporated milk. Stir often until the sugar is dissolved and the butter is melted.
Cook the Milk and Sugar Mixture: Attach a candy thermometer to the pan without letting it touch the bottom. Bring the mixture to a boil, stirring constantly. Continue cooking until the mixture reaches 234°F (soft-ball stage).
Remove from Heat and Mix in Chips & Marshmallows: Once the fudge mixture reaches soft-ball stage, immediately remove the mixture from the heat. Add the chocolate chips and marshmallows and continue stirring until fully melted and smooth. It’s okay if the marshmallows leave streaks of white but it should be mostly melted in with the chips.
Add Remaining Ingredients: Stir in vanilla extract (ensure you add a full teaspoon). Then add your choice of mix-ins, totaling no more than about ¾ cup. Mix well with a sturdy wooden spoon to fully incorporate all ingredients.
Pour and Top: Using a sturdy heat-resistant bowl scraper/spatula, pour the mixture into the prepared baking pan and spread it evenly. If adding sea salt or a dusting of chopped nuts or Oreos, do so now, when the top is still warm and soft.
Allow the fudge to cool completely. Once at room temp, you can cover and put in the refrigerator to help set up, about an hour.
Cut and Serve: Once the fudge is cool and very firm, cut it into one-inch squares and serve or store it in an airtight container.