1½ cups fine graham cracker crumbs (from 11 to 12 graham cracker rectangles)
½ cup granulated sugar
1 stick unsalted butter, melted
3 large eggs
⅔ cup granulated sugar
½ cup fresh lime juice (preferably from Key limes)
2 sticks unsalted butter, at room temperature
1 cup granulated sugar
3 tablespoons water
3 large egg whites
1 makrut (kaffir) lime leaf, very finely chopped, or ½ teaspoon lime zest
1. Make the crust: Preheat the oven to 350°. In a medium bowl, stir together the graham cracker crumbs and the sugar. Pour the melted butter over the mixture and stir until the crumbs are moistened and look like wet sand. Transfer the mixture to a 9-inch pie plate and use the bottom of a measuring cup to compact the crust across the bottom and up the sides of the pan. Bake until the crust is golden-brown, about 6 to 8 minutes (don't be alarmed if the crust shrinks a bit). Remove the piecrust from the oven and set aside to cool.
2. Make the filling: In a medium saucepan set over medium heat, whisk together the eggs, sugar and lime juice. Cook the mixture over medium heat, whisking constantly, until it thickens, about 5 minutes. Turn off the heat and transfer the filling to a heat-safe container. Press a piece of plastic wrap flush over the curd so a skin doesn't form. Once the curd is completely cooled, after about 1 hour, remove the plastic wrap and use an immersion blender (or a stand mixer or food processor) to incorporate the butter 1 tablespoon at a time, mixing well between additions, until all of the butter is added and the lime curd is completely smooth. Scrape the curd into the prebaked pie shell, press a piece of plastic wrap flush over the top so a skin doesn't form, and refrigerate until set, at least 4 hours or preferably overnight.
3. Make the meringue: In a small saucepan set over medium heat, add the sugar and water and stir carefully (try not to splatter the mixture up the sides of the saucepan) until the mixture resembles wet sand. Cook, without stirring, until the sugar-water mixture reaches 240° on an instant-read thermometer, about 15 minutes. The sugar-water mixture will turn syrupy and golden-brown in color.
4. While the sugar mixture cooks, whip the egg whites: To the bowl of a stand mixer fitted with the whisk attachment, add the egg whites and whip on medium-high speed until they form soft peaks, about 3 to 4 minutes. While whipping, slowly pour in the sugar-water mixture, continuing to whip until the meringue holds stiff peaks, 6 to 7 minutes. Add the chopped makrut (kaffir) lime leaf or lime zest and beat until incorporated.
5. Remove the pie from the refrigerator and discard the plastic wrap. Use a rubber spatula to top the lime curd with the meringue, using an offset metal spatula or butter knife to spread the meringue and create soft and pointy peaks.
6. Adjust an oven rack to the upper-middle position and preheat the broiler to high. Place the pie in the oven and broil until the meringue browns, 1 to 3 minutes (watch the meringue closely, as broiler intensities vary); or use a handheld torch to caramelize the meringue. Zest the rind of the lime over the pie and serve.