Step 1: Prepare the Graham Cracker Crust
Preheat oven to 350°F (175°C).
In a medium bowl, mix graham cracker crumbs, sugar, and melted butter until evenly combined.
Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish.
Bake for 8–10 minutes, then cool completely.

Step 2: Make the Key Lime Cheesecake Layer
In a medium mixing bowl, beat softened cream cheese until smooth and creamy.
Add sweetened condensed milk, key lime juice, zest, and vanilla extract.
Beat until the mixture is smooth and slightly thickened.
Pour into the cooled crust and spread evenly.
Place in the refrigerator while you prepare the next layer.

Step 3: Prepare the Lime Mousse Layer
In a small bowl, stir together the unflavored gelatin and water. Let sit for 5 minutes to bloom.
Meanwhile, whip the heavy cream and powdered sugar in a chilled bowl until soft peaks form.
Heat the bloomed gelatin in the microwave or on the stovetop for just a few seconds until fully dissolved (do not boil).
Stir the dissolved gelatin into the key lime juice and zest.
Gently fold this mixture into the whipped cream until smooth and combined.
Carefully spread the mousse layer over the cheesecake layer.
Chill in the refrigerator for at least 4 hours, or overnight, until fully set.

Step 4: Make the Whipped Cream Topping
In a chilled bowl, whip the heavy cream, powdered sugar, and vanilla extract until medium-stiff peaks form.
Dollop or pipe on top of the chilled pie.
Garnish with lime zest or thin lime slices if desired.

Step 5: Slice and Serve
Use a warm, clean knife to cut neat slices.
Serve chilled and enjoy every creamy, citrusy, and crunchy bite!