Reduce The Oven Temp & Check Your Ingredients. If you haven't already, reduce the oven temperature to 325 degrees F. Make sure all your ingredients are room temperature, or they won't blend well. Don't rush this step!
Bring A Pot Of Water To A Boil while you mix up the cheesecake. You'll use this to make a hot water bath for the cheesecake, or at least to fill up a pan of water to place beneath the cheesecake.
Beat Cream Cheese & Sugar. To the bowl of a stand mixer or in a large mixing bowl, combine softened cream cheese and sugar. Mix for 2-3 minutes, scraping down the sides of the bowl occasionally, until the mixture is completely smooth with no lumps.
Mix in the eggs, one at a time on low speed, fully incorporating each egg before adding the next one. Scrape down the sides of the bowl and add sour cream, cornstarch, lemon zest, and vanilla. Mix on low speed until just combined.
With the mixer on low speed (or by hand), gently mix in the heavy cream. The filling should be VERY smooth with no noticeable lumps.
Pour the cheesecake filling into the cooled graham cracker crust. It'll nearly fill the pan to the top. Gently smooth the surface into an even layer or gently tap on the counter a few times to help any air bubbles release.
Prepare Your Hot Water Bath. Add about 3/4 inch of boiling water to your water bath pan (see notes below if you don't have a pan large enough!) Carefully, lower your cheesecake pan into the hot water bath so that the water comes up about 1-inch on the sides of the cheesecake.
Bake The Cheesecake. Carefully transfer the cheesecake to the oven and bake at 325 degrees F. for 60-70 minutes, or until the edges look just dry and the center wobbles like jello. Err on the side of UNDERbaking rather than OVERbaking.
Cool In The Oven For 1 Hour. Turn off the oven and crack the oven door slightly. Let the cheesecake cool in the oven with the door cracked open for 1 hour.
Chill 4-5 Hours. Transfer the cheesecake to the refrigerator (uncovered) and let the cheesecake chill at least 4-5 hours to cool and set completely.