Preheat oven to 375 degrees. Prepare a 9x9x2-inch baking pan by lining it with aluminum foil and spraying the foil with nonstick cooking spray.
Place the butter in a medium microwave-safe bowl and microwave it until melted. Once melted, stir in the sugar and graham cracker crumbs.
Scatter the crumbs in the bottom of the prepared pan, and press them into an even layer. Bake the crust for 5 minutes. Remove from the oven and cool while you prepare the rest of the fudge.
Place the white chocolate, sweetened condensed milk, and salt in a large microwave-safe bowl, and microwave it in 30-second increments, stirring after every 30 seconds. Continue to microwave until the white chocolate is completely melted and your mixture is smooth.
Immediately add the lime juice and lime zest to the white chocolate and stir to combine.
Pour the fudge into the prepared pan and smooth it into an even layer on top of the graham cracker crust. Refrigerate the fudge until it is completely set, for about 4-hours or overnight.
Once set, remove it from the pan using the foil as handles. Use a large sharp knife to cut it into 1-inch squares. It helps to rinse the knife in hot water periodically to get cleaner cuts.
Store Key Lime Pie Fudge in an airtight container in the refrigerator for up to two weeks.
Cook's Note: For best taste and texture, bring fudge to room temperature before serving.