Prep the crust: In a medium bowl, mix graham cracker crumbs, sugar, and melted butter until it looks like wet sand. Press firmly into an 8x8-inch baking pan lined with parchment. Freeze while you make the filling.
Make the filling: Beat cream cheese until smooth. Add sweetened condensed milk, key lime juice, and zest. Mix until silky and fully combined. Gently fold in the whipped topping.
Assemble and freeze: Spread the filling over the chilled crust and smooth the top. Cover and freeze for at least 6 hours, or overnight for best texture.
Serve: Let the bars sit out for 10 minutes before slicing. Garnish with extra whipped topping and lime zest if you’re feeling fancy.
Notes
Want it tangier? Add an extra tablespoon of lime juice.
No whipped topping? Fold in 1 cup of freshly whipped cream.