Line an 8x8-inch square baking dish with parchment paper, allowing some overhang for easy removal later.
In a medium saucepan, combine the semi-sweet chocolate chips and sweetened condensed milk. Heat over medium-low heat, stirring continuously until the chocolate is completely melted and the mixture is smooth.
Remove the saucepan from the heat. Stir in the vanilla extract and salt until well incorporated.
Gently fold in the mini marshmallows and crushed graham crackers (and nuts, if using) until evenly distributed.
Pour the fudge mixture into the prepared baking dish. Spread it evenly with a spatula.
Refrigerate the fudge for at least 3 hours, or until set.
Once set, lift the fudge out of the dish using the parchment paper overhang. Cut into squares and serve.
Notes
Arrange the fudge squares on a decorative plate, and sprinkle some extra crushed graham crackers on top for a finishing touch. You can also drizzle some melted chocolate on top for added flair!