In a medium saucepot over medium high heat, whisk together water, sugar, and cream of tartar until syrup comes to a boil.
Clip a candy thermometer onto the side of the pot and boil, undisturbed, for 10 minutes or until syrup reaches 310 F (hard crack stage).
While the syrup is boiling, combine heavy cream with butter and vanilla and hold aside.
Once sugar syrup reaches 310 F., immediately whisk in cream mixture, return mixture to a boil then reduce heat to medium. Boil, undisturbed, 10-12 minutes or until caramel reaches 240°F (soft ball stage) on the candy thermometer.
Remove pot from heat and pour over popcorn in a very large greased or oiled bowl. Use a greased spatula to thoroughly mix the caramel and corn together then use greased gloved hands to firmly press about an ounce of the caramel popcorn into a greased star mold. Unmold then work quickly through the rest of the popcorn until completed.
Store in an airtight tin for up to 1 week.