For the crust, you will need:
1/3 cup unsalted butter, melted.
1/4th cup sugar
1 cup crushed graham crackers, crushed (about 5 crackers)
For the fudge:
One 14oz can condensed milk
3 cups white chocolate chips
6 limes, washed in hot soapy water
Start by working on the crust. Preheat your oven to 350 degrees. As it is heating, mix the butter, sugar, and graham crackers together. Line a 8x8 cake tin or glass pan with parchment paper (to prevent sticking) and use your hands to pat the crust into a single layer. It is ok if the parchment paper is a bit finicky around the corners, don't worry to much about it, the final product will still be great. Bake for 4-5 minutes until golden. Remove from the oven and let cool.
As the crust is cooling we can start working on the fudge. I like to begin with the limes so that I can focus on the chocolate without distraction. Zest 5 of the limes and put the zest in one pile. You can also go ahead and halve 4 of the limes to juice in a few minutes. Zest the sixth lime and reserve it separately, this will go on the top of the fudge. Over medium heat (or medium low if your stove runs hot) add the condensed milk and the chocolate chips into a sauce pan. Stir together until it is melted and smooth. Add the zest of the five limes, the juice of 4 aggressively squeezed limes. Stir to combine.
Pour this mixture over the cooled crust. Top the fudge with the remaining lime zest, and then cover and refrigerate overnight or until firm. When firm, cut into cubes of your desired size and serve. You may want to cut a thin slice around the edges first to get an even cube if you wish.