In a small bowl, whisk together the egg yolks and lime zest until the mixture has a light green tint.
Whisk in the condensed milk until smooth and then whisk in the lime juice.
Cover and place in the refrigerator for 15 minutes to allow it to thicken.
Preheat the oven to 325 degrees F. and using a food processor add in the graham crackers and process until you have a fine crumb mixture.
Add the melted butter, sugar, and continue to pulse until incorporated and the mixture clumps together.
Place the crust mixture into a 6-inch pie plate and press in the bottom and sides in a flat even layer.
Bake the crust only until it is lightly browned and fragrant, about 15 minutes.
Remove the crust from the oven and while still warm add the filling.
Place the pie in the oven and bake for 15 to 20 minutes, until firm around the edges.
Transfer the pie to a wire rack and allow to cool completely.
Place in the refrigerator to allow the pie to become chilled and set.
Serve with whipped topping.