In a medium saucepan over medium heat, whisk together the pumpkin puree, milk, sweetened condensed milk, and pumpkin spice. Stir occasionally and let heat until it comes to a gentle simmer, do not boil.
4 cups whole milk, 6 ounces sweetened condensed milk, ¼ cup canned pumpkin puree, 1 teaspoon pumpkin pie spice