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