In a medium bowl, combine the butter and crumbs. Press the crust mixture into the bottom and up the sides of a 9-inch pie plate. Bake the crust for 5 minutes. Remove from the oven and let cool while you prepare the filling.
PREPARE THE FILLING:
In a medium bowl, use an electric mixer to beat the egg yolks. Add the zest, juice and condensed milk. Blend well, scraping the sides and the bottom, until all of the ingredients are incorporated and smooth.
Pour the filling into the crust and bake for 15 minutes. Refrigerate the pie for several hours (or overnight) before serving. It will thicken more as it chills. Serve with whipped cream and lime slices for garnish.
Notes
*For the key lime juice, I like to use Nellie & Joe's bottled lime juice. It can usually be found in the juice aisle of your grocery store.