In a medium bowl, stir together the heavy cream and sour cream. Set aside in a warm place for about an hour to thicken.

In a separate bowl, beat eggs with sugar using an electric mixer until pale. Stir in the milk and cinnamon, and transfer to a saucepan. Bring to a simmer over medium-low heat. Cook, stirring constantly, until thick enough to coat the back of a metal spoon. Stir in the vanilla, and remove from the heat. Set aside to cool.

When the custard has cooled, stir in the sour cream mixture. Freeze in an ice cream maker according to the manufacturer's instructions.