Preheat oven to 350 degrees and completely line a 9" square pan with foil or parchment paper. Spray with cooking spray.
In a medium bowl, combine graham cracker crumbs, sugar, and butter until it's fully combined and the texture of wet sand. Pour into prepared pan and press into an even layer using the bottom of a measuring cup or hands. Bake for 6-8 minutes or until crust starts to turn golden brown. Cool completely.
Once crust has cooled, make the filling. In a microwave safe bowl, add the sweetened condensed milk, white chocolate chips, and white chocolate bar, and salt. Heat in 30 second intervals in the microwave stirring in between until chocolate is melted and smooth.
Stir in the lime zest and lime juice. Pour over cooled crust. Chill fudge in the refrigerator for 1-2 hours or until firm. Slice into small squares and serve.
Fudge will keep in the fridge for at least a week in an airtight container.