make the crust preheat oven to 375. using a food processor, crush the graham crackers into crumbs. melt the butter and add it to the graham cracker crumbs along with the sugar and cinnamon. press crumbly mixture into 9 inch pie plate. bake at 375 for 7 minutes. do not overtake, or the crust will be too hard to slice. cool at room temperature before filling
make the filling: combine the sweetened condensed milk, key lime juice, egg, and sour cream. make sure to blend well using a whisk until there are no bits of egg yolk visible. pour filling into a bowl, cover, and chill in fridge while the crust cools.
assemble and bake Preheat the oven to 350. Pour the lime filling into the graham cracker crust and smooth with a wooden spoon or spatula. Bake for 10 minutes, until filling is set.. Cool at room temperature then refrigerate at least 3 hours.
Make the whipped topping. In the bowl of an electric mixer fitted with the whisk beaters, combine the heavy cream, sugar, vanilla and cream of tartar. Beat at high for 1-3 minutes until whipped. Top pie with whipped cream topping by either spreading out gently with a spatula or piping on top. serve cold.