1. Preheat oven to 375 degrees.

2. Combine the cracker crumbs and butter in a medium bowl, and mix until moist. Evenly press mixture into a 9-inch pie plate, and bake until lightly browned, about 12 minutes. Remove from oven, and transfer to a wire rack until completely cooled.

3. In a medium heatproof bowl, whisk the yolks and lime juice together until combined. Set bowl over a saucepan of simmering water, and cook, whisking constantly, until mixture is foamy and registers 160 degrees on an instant-read thermometer, about 6 minutes. Remove bowl from heat, and whisk in condensed milk and zest until well combined. Pour into cooled pie shell and smooth top with a rubber spatula. Freeze until firm, 3 hours.

4. Remove pie from freezer 10 minutes before serving. Combine cream and sugar in the bowl of an electric mixer fitted with the whisk attachment. Whisk on medium speed until soft peaks form, 2 to 3 minutes. Spread evenly over pie, and serve immediately.

5. Remove pie from freezer 5 minutes before serving. Spread whipped cream or meringue evenly over pie. If using meringue, brown the top with a small kitchen torch, or carefully under the broiler. Serve immediately.