Add the cream cheese, sugar, and cornstarch to a large mixing bowl (or to a stand mixer fitted with the paddle attachment.)
Use a hand mixer (or stand mixer) to beat the ingredients together for 2 minutes, until no lumps remain.
Add the two eggs into the cream cheese, and beat the mixture on low speed *just* until the eggs have been fully incorporated.
Add in the heavy cream, vanilla, and lemon juice, and again beat on low speed just until the mixture is cohesive.
Tap the bowl with the cheesecake filling on the counter a few times to get rid of any air bubbles.
Pour ½ of the cheesecake filling over the baked brownie. Use ½ of the remaining brownie batter to dollop over the cheesecake, then use a knife to gently swirl the brownie batter into the cheesecake.
Gently pour in the remaining cheesecake batter, smoothing it into an even layer. Dollop the remaining brownie batter over the cheesecake, and swirl it again.
Fill a 10" cake pan, or any larger pan that can reasonably fit the 7" springform inside of it, a third of the way with hot water. Wrap tin foil around the bottom of your springform pan to avoid the possibility of water seeping into the springform.
Place the springform pan into the larger pan, then place the whole thing into the oven. Bake for 60-70 minutes, or until the cheesecake only slightly wobbles and reaches an internal temperature of at least 155°F / 68°C. If the cheesecake starts to brown while baking, gently place a piece of tinfoil over the top of the pan.
Once the cheesecake is fully cooked, turn off the oven and leave the door slightly ajar with the cheesecake in it for 1 hr. before transferring it to the fridge to chill for at least 8 hours.