Unwrap and break apart the Hershey bars and place them in a bowl along with the chocolate chips. Open up the marshmallow creme and place a spoon into the container, and then measure out your vanilla and pecans (if using). Spray your baking dish with non-stick spray, and set it directly next to your stovetop cooking area.
Combine the sugar, butter, and evaporated milk in a heavy-bottomed pot outfitted with a candy thermometer. Bring the mixture to a boil over medium-high heat, stirring constantly. As the mixture cooks, it will take on a frothy consistency. Keep a close eye on your thermometer, because when it hits 235 degrees (soft ball stage), you will immediately remove the pan from the heat.
Add the chocolate, marshmallow fluff, vanilla, and pecans to the sugar mixture, and stir vigorously until everything is fully incorporated.
Working very quickly before the fudge begins to set, pour it into the prepared baking dish, and spread it out evenly. Set the fudge aside to cool completely (at least 3 hours) before cutting it into squares.