Preheat oven to 350°
Lightly grease a 9-inch round pie dish
In a large bowl, combine graham cracker crumbs, sugar, and cinnamon
Add butter and stir until combined
Pour the crust mixture into the prepared pie dish, pressing it down into the bottom of the dish and halfway up the sides
Bake for 7 minutes, then remove crust
In a large bowl, whisk together condensed milk, sour cream, egg yolks, lime juice, and vanilla until smooth
If desired, whisk in lime zest
Pour the key lime pie filling into the warm pie crust
Return to the oven and bake for 20-25 minutes or until the edges of the filling look set and it’s slightly jiggly
Remove from the oven and set the pie on a wire rack to cool
Chill for at least 3 hours (or overnight)
Once chilled, decorate the pie with homemade whipped cream, if desired, and chill until the whipped cream is set