How to Make It
Preheat the oven to 350°. In a medium bowl, stir together the graham-cracker crumbs, melted butter and cinnamon. Evenly press the crumbs into a 9-inch pie plate. Bake for about 12 minutes, or until firm. Remove the crust from the oven and lower the temperature to 325°.
In a large bowl, whisk the egg yolks with the cream of tartar until frothy. Stir in the condensed milk, then whisk in the Key lime juice until fully incorporated.
Pour the filling into the crust and bake for about 15 minutes, or until set. Let the pie cool on a rack, then refrigerate until chilled before serving.