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