01 - Line an 8x8-inch baking pan with parchment paper, allowing overhang on opposite sides for easy removal later.
02 - In a medium saucepan over low heat, combine white chocolate chips, sweetened condensed milk, and butter. Stir constantly until completely smooth and melted, about 5 minutes.
03 - Remove saucepan from heat. Stir in vanilla extract and pinch of salt until fully incorporated.
04 - Let mixture cool for 2-3 minutes to prevent marshmallows from melting. Gently fold in 1 cup of Lucky Charms marshmallows until evenly distributed.
05 - Pour fudge mixture into prepared pan. Use spatula to spread evenly into corners and smooth the top surface.
06 - Sprinkle remaining ½ cup marshmallows evenly over fudge surface. Gently press down with spatula to adhere without pressing them completely into the mixture.
07 - Refrigerate for at least 2 hours or until completely firm and set throughout.
08 - Lift fudge from pan using parchment overhang. Cut into 16 equal squares with a sharp knife.