Preheat the oven to 350°F.
Grease a 9-inch springform pan and line the bottom with parchment paper.
In a mixing bowl, combine brownie mix, eggs, water, and oil and prepare according to the instructions on the box.
Transfer brownie batter to the prepared springform pan and bake for 15 minutes until the brownies are set across the top.
Meanwhile, beat the cream cheese with a hand mixer or stand mixer until light and fluffy.
Add the granulated sugar and beat until combined.
Add flour, vanilla extract, sour cream, and ¼ cup heavy cream and mix to combine.
Add in one egg at a time and stir by hand until mixed in.
Pour the cheesecake mixture over the baked brownie. The brownie crust does not need to be cooled after 15 minutes of baking.
Reduce the oven temperature to 325°F.
Return the springform pan to the oven and bake for 45 minutes. After 45 minutes, turn off the oven, keeping the door closed for 20 minutes. Then, crack the door of the oven and allow the cheesecake to cool for an additional 30 minutes.
Remove cheesecake from the oven and cool to room temperature, and then chill in the refrigerator for 2 to 3 additional hours.
Just before serving, whip the remaining 1 cup of heavy cream with the powdered sugar until stiff peaks form.
Top cheesecake with whipped cream and a drizzle of hot fudge sauce.