You may accompany the squares with a raspberry coulis.
4 tablespoons (1/2 stick) unsalted butter, melted and cooled, plus more for pan
2/3 cup shelled pistachios
1 cup (4 ounces) graham-cracker crumbs
1/4 cup sugar
1 tablespoons grated lime zest
FOR THE FILLING
2 large egg yolks
1 can (14 ounces) sweetened condensed milk
1/4 cup fresh lime juice
1/4 cup fresh lemon juice
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.
In a food processor, finely chop 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.
In a large bowl, whisk together egg yolks and condensed milk. Add lime and lemon juices; whisk until smooth. Pour filling into cooled crust; carefully spread 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 a cutting board. With a serrated knife, cut into 16 squares, wiping knife with a damp kitchen towel between each cut.