Preheat your oven to 350°F. Grease a 9-inch springform pan and line the bottom with parchment paper.
In a mixing bowl, combine the brownie mix, 2 eggs, water, and vegetable oil, then prepare the brownie batter according to the package instructions. (If the box instructions vary, follow the ingredient portions and directions provided on the package.) Once prepared, pour the brownie batter into the prepared springform pan and bake for 15 minutes, or until the top of the brownies is set.
While the brownies are baking, beat the cream cheese using a hand mixer (or stand mixer) until smooth and fluffy. Add the granulated sugar and continue beating until combined. Next, add the flour, vanilla extract, sour cream, and ¼ cup of heavy cream, and mix until fully incorporated.
Add one egg to the mixture and stir by hand until combined, then add the remaining egg and mix again. Once the cheesecake mixture is ready, pour it over the baked brownie layer (there’s no need to let the brownie cool after the initial 15-minute baking time).
Reduce the oven temperature to 325°F, then return the springform pan to the oven and bake for an additional 45 minutes. After 45 minutes, turn off the oven but keep the door closed for 20 minutes. Then, crack the oven door slightly and allow the cheesecake to cool for another 30 minutes.
Remove the cheesecake from the oven and let it cool to room temperature, then chill in the refrigerator for 2-3 hours.
Just before serving, whip the remaining 1 cup of heavy cream with powdered sugar until stiff peaks form. Top the cheesecake with whipped cream and a drizzle of hot fudge sauce for a perfect finishing touch.