1. Preheat oven to 350 farenheit.
2. Brush an 8 inch square baking pan with melted butter.
3. Line bottom with parchment paper, leaving a 2 inch overhang on two sides.
4. In a food processor, finely grind pistachios with graham cracker crumbs, sugar and zest.
5. Blend in butter.
6. Press mixture into bottom and 1/2 inch up sides of prepared pan.
7. Bake until lightly browned, 8 - 10 minutes. Cool crust, 30 minutes.
8. In a large bowl, whisk together egg yolks and condensed milk.
9. Add lime juice and whisk until smooth.
10. Pour filling into cooled crust, carefully spreading to edges.
11. Bake until set, about 15 minutes.
12. Cool in pan on wire rack then chill at least 1 hour before serving.
13. Using parchment paper overhang, lift out of pan and transfer to a cutting board.
14. Using a serrated knife, cut into 16 squares, wiping knife with a damp kitchen towel between each cut.