01 - Line an 8x8-inch baking pan with parchment paper, allowing overhang on opposite sides for easy removal later.
02 - Combine white chocolate chips, sweetened condensed milk, and butter in a medium saucepan over low heat. Stir constantly until completely smooth and melted, about 5 minutes.
03 - Remove saucepan from heat. Stir in vanilla extract and salt until fully incorporated.
04 - Let the mixture cool for 2-3 minutes to prevent marshmallows from melting completely. Fold in 1 cup of Lucky Charms marshmallows until evenly distributed.
05 - Pour the fudge mixture into the prepared pan. Use a spatula to spread it into an even layer.
06 - Sprinkle the remaining ½ cup of Lucky Charms marshmallows over the surface. Gently press them into the fudge to adhere.
07 - Refrigerate for at least 2 hours or until the fudge is completely set and firm to the touch.
08 - Lift the fudge from the pan using the parchment overhang. Cut into 16 equal squares using a sharp knife dipped in hot water and wiped dry for clean cuts.