Line a 9x13-inch or 11x15-inch baking pan with parchment paper, ensuring the paper extends over the edges for easy removal. Lightly grease the parchment with butter or non-stick spray. Set aside.

In a heavy-bottomed saucepan or Dutch oven, add the granulated sugar, marshmallow fluff, evaporated milk, butter, and salt. Heat over medium-low, stirring continuously, until the sugar is completely dissolved, and the ingredients are well combined.

Increase the heat to medium, stirring frequently, and bring the mixture to a full boil. Allow it to boil while stirring constantly until the temperature reaches 234–240°F, about 5 minutes. Use a candy thermometer for accuracy.

Remove the pot from the heat. Stir in the semi-sweet chocolate chips and vanilla extract until the chocolate is fully melted and the mixture is smooth. Add the black gel food coloring gradually, mixing well until the desired coal-black color is achieved.

Pour the fudge mixture into the prepared baking pan, spreading it evenly with a spatula. Smooth the surface for a uniform finish.

Let the fudge cool and set at room temperature for at least 1 hour.

Once the fudge is firm, lift it out of the pan using the parchment paper overhang. Cut into small squares or irregular shapes to resemble lumps of coal.