In a medium bowl, mix together graham cracker crumbs, sugar and butter until well blended. (If using packaged graham crackers, crush them in a plastic bag with a rolling pin or chop in food processor to get fine crumbs).
Press crumbs on bottom and up sides of 9-inch pie pan. Bake at 350 degrees Fahrenheit for about 8 minutes. Let cool before adding filling.
For Filling
Squeeze juice from about 22 key limes to reach ¾ cup key lime juice (alternate, use ¾ cup bottled key lime --NOT LIME-- juice into a medium-sized mixing bowl.
Add lime zest, 2 cans sweetened condensed milk, egg yolks and lemon juice, and mix with electric or wire whisk mixer for 2-3 minutes or until well blended and creamy.
Pour filling into baked graham cracker crust and bake in oven for 15-20 minutes at 350 degrees (check pie after 15 minutes). Pie should jiggle when you remove it. Don't let it get brown.
Let pie cool, then refrigerate for at least 3 hours Serve pie cold topped with whipped cream.