Preheat oven and make cracker crumbs: Preheat oven to 350°F. Process graham crackers and 2 tablespoons sugar in a food processor until fine crumbs form.
Add butter and process: Add butter. Process until combined.
Form crust in tart pan and bake: 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.
Make filling: Meanwhile, in a medium bowl, combine condensed milk, lime juice, egg yolks, remaining ½ cup sugar, and salt; whisk until smooth.
Pour filling into crust and bake: Pour mixture into crust (warm or cool is fine), leaving ⅛ 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 then refrigerate: Cool completely at room temperature; then refrigerate until chilled, at least 2 hours. Serve, garnished with lime zest and whipped cream, if desired.
