01 - Line an 8x8-inch baking pan with parchment paper, leaving overhang on opposite sides for easy removal later.
02 - In a medium saucepan over low heat, combine semi-sweet chocolate chips, sweetened condensed milk, and butter. Stir constantly until completely melted and smooth, then remove from heat.
03 - Let chocolate mixture cool for 2–3 minutes to prevent marshmallows from melting completely. Gently fold in mini marshmallows, chopped nuts, and optional milk chocolate chips until evenly distributed.
04 - Pour mixture into prepared pan, using a spatula to spread evenly and smooth the surface. Refrigerate for at least 2 hours until completely firm.
05 - Lift fudge from pan using parchment paper overhang. Cut into 16 equal squares with a sharp knife and serve immediately, or store refrigerated in an airtight container.