Combine melted butter and sugar in a medium bowl. Stir in crushed graham crackers.
Spread evenly into 9-inch pie plate and press onto bottom and sides to form an even crust. The bottom of a measuring cup works great for pressing the crumbs firmly into the pie plate.Chill for an hour until firm OR bake at 375-degrees for 5 min and allow to cool before filling.
For the filling:
Preheat oven to 350-degrees. Zest key limes until there are 2 teaspoons of zest, then juice them. Pour the fresh key lime juice into a 1/2 cup measuring cup.Cut zested key limes in half and juice them.Pour the key lime juice into a 1/2 cup measuring cup. If the 1/2 cup isn't filled, add key lime juice to fill it completely.
Beat the egg yolks until they are thick and turn to a light yellow. Use a stand mixer or an electric mixer. Slowly add sweetened condensed milk while mixing on a low speed.
Gradually add the key lime juice and zest and continue to mix on a low speed just until blended.
Pour the mixture into the prepared pie shell and bake 15 minutes. Remove and let cool.
Serve with a garnish of whipped topping and a small slice of lime if desired. Store pie in refrigerator until ready to serve
*If key limes and/or key lime juice are not available, you can still make this pie! Substitute 1 lime in place of the 6 key limes.