Preheat the oven to 350°F (175°C). Grease a 9-inch pie dish with butter or cooking spray.
In a medium bowl, combine graham cracker crumbs, cinnamon, sugar, and melted butter. Mix until everything is well combined.
Press the mixture into the greased pie dish, making sure to press down firmly and evenly. Bake in preheated oven for 10 minutes. Allow to cool completely before filling.
In a medium bowl, beat the egg whites until they form stiff peaks. Set aside. In a separate bowl, combine grated lime peel (lime zest), lemon juice, key lime juice, sugar, and sweetened condensed milk.
Mix until everything is well combined. Gently fold in the egg whites. Pour the filling into the cooled crust and spread evenly with a spatula or spoon.
Bake for 25-30 minutes, or until the edges are lightly golden brown. Allow to cool before topping with whipped cream.
In a large bowl, beat together heavy whipping cream and vanilla extract until soft peaks form. Add the powdered sugar and continue to beat until stiff peaks form.
Spread the whipped cream over the cooled pie with a spatula or spoon. Refrigerate for at least 2 hours before serving.