Begin by lining a 9x9-inch baking pan with parchment paper, leaving some overhang on the sides.
In a medium saucepan over low heat, combine the semi-sweet chocolate chips, sweetened condensed milk, and butter. Stir continuously until smooth.
Once the chocolate mixture is silky smooth, remove from heat and gently fold in the mini marshmallows and chopped walnuts.
Pour the fudge mixture into the prepared baking pan and spread it evenly. Tap the pan gently on the counter.
Sprinkle the remaining mini marshmallows and chopped walnuts on top, pressing them down lightly.
Place the fudge in the refrigerator for at least 2 hours, or until firm to the touch.
Using a sharp knife, cut the fudge into squares or rectangles and serve immediately or store for later.
