Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
Prepare Brownie Layer: In a mixing bowl, combine the brownie mix, ½ cup vegetable oil, 3 tablespoons water, and 2 eggs. Follow the instructions on the box for mixing. Pour the brownie batter into the prepared springform pan and bake for 15 minutes, or until the top is set.
Prepare Cheesecake Layer: While the brownie layer is baking, beat the softened cream cheese with a hand mixer or stand mixer until light and fluffy. Add the granulated sugar and continue beating until fully combined.
Add Remaining Ingredients: Mix in 1 tablespoon of flour, 1 teaspoon of vanilla extract, ¼ cup of sour cream, and ¼ cup of heavy whipping cream until smooth. Add the eggs one at a time, mixing by hand until just incorporated after each addition.
Pour Cheesecake Over Brownie: Once the brownie layer has baked for 15 minutes, remove it from the oven (no need to cool it). Reduce the oven temperature to 325°F (165°C). Pour the cheesecake mixture over the brownie layer, spreading it evenly.
Bake Cheesecake: Return the springform pan to the oven and bake for 45 minutes. After 45 minutes, turn off the oven but leave the cheesecake inside with the door closed for an additional 20 minutes. After 20 minutes, crack the oven door and allow the cheesecake to cool inside for another 30 minutes.
Cool and Chill: Remove the cheesecake from the oven and allow it to cool to room temperature. Once cooled, refrigerate for 2-3 hours or until completely chilled.
Prepare Whipped Cream: Just before serving, whip 1 cup of heavy cream with 3 tablespoons of powdered sugar until stiff peaks form.
Serve: Top the cheesecake with the whipped cream and drizzle with hot fudge sauce or chocolate ganache. Slice and enjoy!