Lower the oven temperature to 250°F, and place a sheet pan halfway filled with water in the oven.
With your stand mixer and paddle or hand mixer and beaters, cream the cream cheese until soft and creamy. Scrape down the bowl and attachments, and mix on the highest speed for high for 60 seconds. Lower the speed to medium low.
Whisk together the cornstarch and sugar then add to the cream cheese. Mix until fully incorporated with no lumps. Scrape down the bowl and attachments, and mix on the highest speed for high for 60 seconds. Lower the speed to medium low.
Add the eggs and yolk and mix until fully incorporated with no lumps. Scrape down the bowl and attachments, and mix on the highest speed for high for 60 seconds. Lower the speed to medium low.
Add the sour cream, and mix until fully incorporated with no lumps. Scrape down the bowl and attachments, and mix on the highest speed for high for 60 seconds. Lower the speed to medium low.
Add the pumpkin puree and spice, and mix until fully incorporated with no lumps. Scrape down the bowl and attachments, and mix on the highest speed for high for 60 seconds. Lower the speed to medium low.
Pour into your cake pan and place in the prepared water bath in the oven.
Bake in your prepared water bath for 75-90 minutes or until set or until the internal temperature reaches 150℉. Allow to cool before placing in freezer for a minimum of 4 hours to remove the pan and liner and slice.