Place a cast iron skillet on the stovetop over medium heat. Add the butter and sugar, stirring constantly until the butter melts into the sugar, forming a smooth, creamy mixture.

Pour in the evaporated milk and bring the mixture to a gentle boil, stirring constantly. Once bubbling, lower the heat and simmer for 5 minutes, continuing to stir until the mixture thickens.

Stir in the black walnuts thoroughly, ensuring they are evenly distributed. Continue cooking and stirring for another 3-4 minutes until the fudge is glossy, smooth, and slightly pulls when you lift the spoon.

Remove the skillet from the heat once the fudge reaches a thick but pourable consistency.

Pour the fudge mixture into a prepared pan and let it cool completely at room temperature for a few hours.

Chill the fudge in the refrigerator for at least 2 hours to set firmly. Once set, loosen the edges with a butter knife, invert onto a cutting board, remove the parchment paper, and cut into squares.