Preheat oven to 350 degrees if not still preheated from the crust.
In a large bowl, mix the cream cheese, sugar and cornstarch together with hand mixer on medium speed until smooth and free of lumps, about 2 minutes.
24 oz (680 g) cream cheese, softened at room temperature, 1 cup (200 g) granulated white sugar, 1 tbsp (8 g) cornstarch
Add in the sour cream, vanilla and lemon zest to the cream cheese mixture and mix on medium-low speed just until combined.
1/4 cup (62 g) sour cream, at room temperature, 1 tbsp (15 ml) vanilla, 2 tbsp (20 g) lemon zest
Add in the eggs and mix on medium-low speed just until combined.
3 eggs, at room temperature
Pour the cheesecake batter into the prepared baking pan with the crust. Spoon the blueberry jam on top and gently swirl with a butter knife.
Bake for 38-42 minutes. The center will have a slight jiggle. Then turn the oven off and prop the oven door open for 10 minutes using a wooden spoon.
Remove the cheesecake bars from the oven and allow them to cool completely in the pan on a wire rack. Then cover and transfer them to the fridge. Allow them to chill in the fridge for 6 hours, or until completely cold. (I usually bake them the night before and allow them to chill overnight.) Once the cheesecake is cold, pull up on the parchment paper to carefully remove the cheesecake from the pan. Transfer to a serving plate or board. Cut into 16 squares using a sharp knife. (Run the knife under hot water then wipe it with a towel each time you make a cut. This will allow you to make clean, pretty cuts.)