Line an 8x8-inch baking dish with parchment paper, leaving an overhang on the sides for easy removal.
In a medium saucepan, combine the semisweet chocolate chips, sweetened condensed milk, and butter. Heat over low heat, stirring gently until the chocolate is completely melted and the mixture is smooth.
Remove the saucepan from heat and stir in the vanilla extract.
Fold in the mini marshmallows and chopped nuts until evenly distributed throughout the chocolate mixture.
Pour the fudge mixture into the prepared baking dish, spreading it evenly. Press down gently to ensure it is packed tightly.
Refrigerate for at least 2 hours or until the fudge is firm. When set, lift the fudge out of the dish using the parchment overhang and cut into squares.
