9 to 10 graham crackers (each 2 1/2 by 5 inches)
2 tablespoons plus 1/2 cup sugar
4 tablespoons unsalted butter, melted
1 can (14 ounces) sweetened condensed milk
3/4 cup fresh lime juice (from 4 to 6 limes), or fresh or bottled key-lime juice
4 large egg yolks
Slivered lime zest, for garnish (optional)
Preheat oven to 350 degrees. Process graham crackers and 2 tablespoons sugar in a food processor until fine crumbs form; add butter. Process until combined. Transfer mixture to a 9-inch tart pan with a removable bottom; pat into bottom and up sides. Place pan on a baking sheet, and bake until crust is fragrant and slightly colored, about 10 minutes.
Meanwhile, in a medium bowl, combine condensed milk, lime juice, egg yolks, remaining 1/2 cup sugar, and salt; whisk until smooth. Pour mixture into crust (warm or cool is fine), leaving 1/8 inch at the top; return to oven. Bake until filling is set around edge but still slightly loose in center, 20 to 25 minutes.
Cool completely at room temperature; then refrigerate until chilled, at least 2 hours. Serve, garnished with zest, if desired.