Preheat oven to 300°F.
Generously butter the inside of a 10-inch springform pan.
Cut two large pieces of foil and lay them on your work surface in a cross.
Set the springform pan in the middle and fold the edges of the foil up around the sides of the pan.
The foil gives you extra protection against water getting into the pan during the water bath step.
In a large mixer-bowl or bowl, beat the cream cheese and butter until it is smooth and there are no lumps.
Add the sour cream,sugar,cornstarch,lemon juice, lemon zest and vanilla to the bowl and beat on medium-low speed until combined and creamy.
Scrape down the beater and sides of the bowl with a spatula.
Next add the eggs, one at a time, beating slowly until everything is combined.
Pour into the prepared springform pan and place in a roasting pan large enough to prevent the sides from touching.
Bring a few cups of water to a boil and pour the water into the roasting pan, being careful not to splash any water onto the cheesecake.
Fill the pan to reach halfway up the sides of the springform pan.
Bake for 2 hours, 15 minutes, or until the cake is very lightly colored and a toothpick inserted in the center emerges clean.
Turn off the oven and crack the door open. Let the cheesecake cool slowly for two to three hours.
Do not try to remove the cheesecake from the pan until it has set up in the refrigerator overnight or at least 6 hours.
This step is crucial for letting the cheesecake set and achieving perfect cheesecake texture — don't rush it.
Unmold the cake and top the cheesecake just before serving.
You can either serve the cake right from the bottom of the springform pan or garnish with blackberry sauce or whatever topping you desire.