Preheat oven to 350º.
Process graham crackers and 2 tablespoons sugar in a food processor until fine crumbs form.
Add the butter.
Process until combined.
Transfer mixture to an un-greased 9 inch tart pan with a removable bottom.
Pat the crust into bottom and up sides.
Place pan on a baking sheet and bake until crust is slight colored, about 10 minutes.
Meanwhile, in a medium bowl, combine the condensed milk, lime juice, egg yolks, remaining 1/2 cup sugar and 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 the center, 20-25 minutes.
Cool completely at room temperature, then refrigerate until chilled, at least 2 hours.
To store longer, cover and refrigerate, up to 2 days.
Garnish with lime zest.