Preheat your oven to 375 degrees.
Line an 8×8, or 9×9 baking dish with aluminum foil and spray, to evenly coat the foil, with non-stick cooking spray
Either in a microwave, or on the stove-top, melt butter.
When melted combine butter, sugar, and graham crackers in a bowl, stirring until evenly combined and moist.
Transfer graham cracker mix into your foil lined pan, smoothing and spreading to make an even layer.
Bake the crust for 7-10 minutes, until the edges are lightly browned. Remove from oven and let cool.
Melt chocolate, salt, and condensed milk in a stove-pot over medium low heart, stirring until evenly combined.
When mixture is smooth, add lime juice and zest, stirring until evenly combined.
Transfer the fudge to the prepared baking dish, spreading into an even layer over the graham cracker crust.
Refrigerate fudge until it is set, about 2 hours, or overnight.
Using the foil as handles, remove the fudge from the baking dish.
Using a sharp, hot knife (run under hot water periodically to keep hot) cut the fudge into 1 inch squares.
Let the fudge rest at room temperature for 15-20 minutes before slicing & serving.