Make crust: Preheat oven to 350 degrees. Stir together graham cracker crumbs, sugar, and butter in a small bowl. Press evenly onto bottom of a 9x12 baking dish. Bake until dry and golden brown, about 10 minutes. Let cool completely on a wire rack. (Leave oven on.) Make filling: Put eggs and lime zest in the bowl of an electric mixer fitted with the whisk attachment. Mix on high speed until very thick, about 5 minutes. Reduce speed to medium. Add condensed milk in a slow, steady stream, mixing constantly. Raise speed to high; mix until thick, about 3 minutes. Reduce speed to low. Add lime juice; mix until just combined. Spread filling evenly over crust using a spatula. Bake, rotating dish halfway through, until filling is just set, about 10 minutes. Let cool completely on a wire rack. Refrigerate at least 4 hours (or overnight). Dust with powdered sugar. Cut into 2-by-2-inch bars. Set in cupcaker papers if desired. The bars will keep, wrapped in plastic, in the refrigerator for up to three days.