4tablespoons unsalted butter, melted, plus 1 tablespoon a room temperature for greasing the plate
1 1/2cupsfinely ground graham cracker crumbs (from about 10 graham crackers)
6light grates of lemon zest
2cupssweetened condensed milk (use any left over to make caramel; see headnote)
2tablespoons grated lime zest (from about 2 limes)
3/4cupfresh lime juice (from 8-10 limes)
Preheat the oven to 350 degrees. Position the rack in the center of the oven. Grease the bottom and sides of a 9 inch pie plate with the room temperature butter.
Make The Crust:
In a medium bowl, mix together the graham cracker crumbs, sugar, melted butter, and lemon zest. the texture should resemble wet sand. Use he bottom of a glass measuring cup to to press the crust into an even layer over the bottom and up the sides of a pie plate. Place the oven in the pie plate and bake until the crust firms and browns lightly, 10 to 12 minutes. Remove it from the oven and set it aside to cool; keep the oven on.
Make the Filling:
In a medium bowl, whisk together the condensed milk, sour cream, lime zest, and lime juice until smooth. Pour the filling over the cooled pie crust.
Place the pie plate in the oven on the center rack and bake until the filling is somewhat firm in the center, 20 to 22 minutes. Let the pie cool completely at room temperature, then refrigerate it for at least for 1 hour (or cover the cooled pie with plastic wrap and refrigerate for up to 12 hours).
Cut the pie into wedges and serve them with unsweetened whip cream on the side. You can also use dollops of he whipped cream on top of the wedges to accentuate the contrast between the rich cream and sweet-tart filling. The pie can be refrigerated, covered, for up to 3 days.
In the bowl of a stand mixture fitted with the whisk attachment (or in a large metal bowl using a handheld mixer), beat the cream and add the sugar (if using) until the cream reaches stiff peaks.
Use the right away or store in an airtight container in the refrigerator for up to 12 hours. When you are ready to serve it, simply whisk the cream a few times to fluff up the texture.