Whisk egg yolks and lime zest together in medium bowl about one minute.
Whisk in sweetened condensed milk until smooth, then whisk in lime juice.
Cover mixture and set aside at room temperature until thickened, about 30 minutes.
Meanwhile, adjust oven rack to middle position and heat oven to 325 degrees.
Sprinkle butter and sugar over graham cracker crumbs and mix together. Mixture should resemble wet sand.
Press mixture into two 4-¾ inch mini tart pans or one 6-inch pie plate. Use the back of a spoon to press into the sides.
Bake until just beginning to brown, about 10 minutes. It will take a bit longer if using one 6-inch pie plate, about 15 minutes.
Transfer pans to a wire rack to cool, but leave oven on.
Pour thickened filling into warm pie crusts and bake until center is firm but jiggles slightly, about 15 minutes.
Let pie cool on wire rack, about one hour. Cover loosely with plastic wrap and refrigerate until filling is chilled and set, about 3 hours.
Using hand mixture set at medium-low speed, beat cream, sugar and vanilla in medium bowl until foamy, about 1 minute. Increase speed to high and beat until soft peaks form, 1 to 3 minutes. Pipe whipped cream attractively over the top of the chilled tarts and serve.
Recipe from The Complete Cooking for Two Cookbook by America's Test Kitchen.