01 - Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - In a medium saucepan over low heat, combine semi-sweet chocolate chips, sweetened condensed milk, and butter. Stir constantly until melted and smooth. Remove from heat.
03 - Let mixture cool for 2–3 minutes, then fold in marshmallows, nuts, and milk chocolate chips, mixing gently to combine.
04 - Pour mixture into prepared pan, smoothing top with spatula. Refrigerate for at least 2 hours until fully set.
05 - Lift fudge out using parchment paper, cut into squares, and serve.