Prepare the Crust: In a medium bowl, combine graham cracker crumbs, granulated sugar, and melted butter. Use a fork to mix until the crumbs are moist and evenly coated. Spoon the mixture into individual serving cups, pressing firmly down with the back of a spoon to form a solid layer. Set aside.

Make the Filling: In a large mixing bowl, beat the softened cream cheese using a hand mixer until creamy and free of lumps, about 2-3 minutes. Gradually add the powdered sugar and continue mixing until incorporated. Add key lime juice and optional lime zest, mixing well until smooth, about 1-2 minutes.

Fold in Whipped Topping: Gently fold the whipped topping into the cream cheese mixture using a spatula until no streaks remain visible.

Assemble the Cups: Spoon the key lime filling over the prepared crust in each cup, filling them to the top. Smooth the tops for an even finish.

Chill: Cover the cups with plastic wrap and refrigerate for at least 2 hours, allowing the filling to set.

Garnish and Serve: Remove from the refrigerator, garnish with additional whipped topping and lime zest, and serve chilled.