Combine the graham cracker crumbs and granulated sugar in a medium-sized mixing bowl and stir until well combined. Add 6 tablespoons of melted butter and mix until all of the crumbs are moistened. If the mixture seems a little dry, you can melt one more tablespoon of butter and mix it in.

Scoop the graham cracker mixture into a 9 or 9.5-inch pie plate. Use your hands or a measuring cup to firmly press it down into an even layer on the bottom and up around the sides of the dish.

Transfer to the refrigerator to chill while you make the filling.

In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a handheld mixer, beat the cream cheese on medium speed until smooth (about 30 seconds). Add the sweetened condensed milk and continue mixing until fully combined, stopping to scrape down the sides of the bowl as needed.

Add the key lime juice and optional zest and mix until fully combined. If needed, use a rubber spatula to turn the filling a few times to make sure everything is thoroughly mixed together. Set aside.

Add the heavy whipping cream to a separate mixing bowl. Using a handheld mixer or stand mixer fitted with the whisk attachment, mix on low-medium speed for 30 seconds to 1 minute then increase the speed to medium-high and continue mixing until stiff peaks form.

Gently fold one-third of the whipped cream into the cream cheese mixture until just combined, then fold in the remaining whipped cream until fully combined.

Remove the pie plate with the crust from the refrigerator and scoop the filling on top of the crust. Spread the filling into one even layer and smooth it out.

Cover with plastic wrap and transfer to the refrigerator to chill for at least 5 to 6 hours or overnight.

Top with whipped cream and some freshly grated key lime zest, if desired. Slice into pieces and enjoy!