01 - Line an 8x8-inch baking pan with parchment paper, allowing overhang on edges for easy removal later.
02 - Combine semi-sweet chocolate chips, sweetened condensed milk, and butter in a medium saucepan over low heat. Stir continuously until completely melted and smooth, then remove from heat.
03 - Let chocolate mixture cool for 2–3 minutes. Gently fold in marshmallows, nuts, and milk chocolate chips until evenly distributed.
04 - Transfer mixture to prepared pan, using a spatula to smooth the surface evenly.
05 - Refrigerate for at least 2 hours until completely firm. Lift out using parchment paper, cut into 16 squares, and serve.