Preheat the oven to 350°F. Spray a 9" pie pan with non-stick cooking spray and set aside.

Before getting started, if you're using fresh key limes for your pie, go ahead and squeeze them first. 1 lb. fresh key limes yields about 1/2 cup of juice. Strain any pulp or seeds out.

Start by making the crust. If you're using whole Graham crackers, then you need about 10 sheets to yield 1 1/2 cups. Either pulse them in a food processer for a few seconds or place them in a gallon sized Ziploc bag and crush them using a rolling pin.

Mix the Graham cracker crumbs, melted butter and sugar all together in a large bowl until combined and resembling a coarse, sandy mixture. Then, pour the crust mixture into the prepared pie pan and pack the crumbs around the pan and up the sides. I use the back of a small, metal dry measuring cup, but you can use your fingers to do this. Just make sure to pack the crumbs together well.

Bake the crust for 12 minutes. While it's baking, start to prepare the pie filling. Once the 12 minutes are up, remove the crust from the oven, but leave the oven on.

Using either a hand mixer or a stand mixer with a paddle attachment, beat the cream cheese until smooth, about 1 minute. Beat in the egg yolks and scrape down the sides of the bowl as needed.

Beat in the sweetened condensed milk, lime juice and lime zest (if using). Pour the filling into the warm crust.

Bake for 16-18 minutes, or until the sides are very lightly browned. Its ok if the center still has a slight jiggle.

Allow the pie to cool on a wire rack for 30 minutes, then transfer it to the fridge and chill for at least 4 hours, before slicing and serving. Pie should be served chilled.

Keep any leftovers stored, covered, in the fridge for up to 3 says.