Preheat oven to 350°F Brush an 8-inch square baking dish with melted butter. Line bottom with parchment paper, leaving a 2-inch overhang on 2 sides.
In a food processor, finely grind pistachios with graham-cracker crumbs, sugar and zest. Blend in butter. Press mixture into bottom and 1-inch up sides of prepared pan. Bake until lightly browned, 8-12 minutes. Cool crust, 30 minutes.
To Make The Filling: In a large bowl, whisk together egg yolks and condensed milk. Add lime juice; whisk until smooth. Pour filling into cooled crust; carefully spreading to edges.
Bake until set, about 15 minutes. Cool in pan on rack then chill at least 1 hour before serving. Using parchment paper overhang, lift out of pan, and transfer to cutting board. With a serrated knife, cut into 16 squares, wiping knife clean with a damp kitchen towel between each cut. To store in refrigerator, cover in plastic wrap.