1. Preheat oven to 350 degrees. Brush an 8-inch square baking dish with melted butter. Line bottom with parchment paper, leaving a 2-inch overhang on two sides.
2. 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 to 12 minutes. Cool crust, 30 minutes.
3. 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 spread to edges.
4. 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 a cutting board.
5. With a serrated knife, cut into 16 squares, wiping knife with a damp kitchen towel between each cut. To store in refrigerator, cover with plastic wrap.

Makes 16.