In a medium bowl combine graham cracker crumbs, melted butter, and sugar. Stir until combined then press tightly into the bottom and up the sides of a 9-inch pie dish. Chill the crust for 30 minutes or longer.
1 ½ cups graham cracker crumbs, 6 tablespoons unsalted butter, ⅓ cup granulated sugar
In the bowl of a stand mixer fitted with the whisk attachment whip up the heavy cream until stiff, then transfer to a bowl and set aside.
1 cup heavy whipping cream
In the same bowl of a stand mixer fitted with the paddle attachment beat cream cheese at low-medium speed until smooth, about 1 minute.
8 oz cream cheese
Add the sweetened condensed milk slowly then beat until smooth and well mixed.
14 oz can sweetened condensed milk
Add the whipped cream and mix until combined.
With the mixer running on low speed, slowly add the key lime juice. The mixture will start to thicken and to get creamy. Continue to mix for 1 more minute. Then add the lime zest if using.
½ cup key lime juice, 1 tablespoon lime zest
Pour into prepared pie dish and refrigerate for atleast 6 hours or overnight until firm.