For the crust
4 tablespoons (1/2 stick) unsalted butter, melted and cooled, plus more for pan
1-1/2 cup (about 24 squares) graham-cracker crumbs
1/4 cup sugar
For the filling
2 large egg yolks
1 can (14 ounces) sweetened condensed milk
1/2 cup fresh lemon juice (i used 3 lemons)
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 grind the graham-cracker squares. Then add in sugar and butter and blend to mix. Press mixture into bottom and 1 inch up sides of prepared pan. Bake until lightly browned, 8 to 12 minutes. Cool crust, 30 minutes.
To make the filling: In a large bowl, whisk together egg yolks and condensed milk. Add lemon juice; 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.