Preheat your oven to 350°F (175°C) and grease a 9-inch springform pan. In a large mixing bowl, whisk together melted unsalted butter and granulated sugar until well combined. Add in the eggs one at a time, mixing smoothly. Gradually incorporate sifted cocoa powder, flour, and a pinch of salt until just blended. Pour the brownie batter into the prepared pan and bake for 15-18 minutes until set but still moist.

Once the brownie base has cooled slightly, reduce the oven temperature to 325°F (160°C). In a separate bowl, beat softened cream cheese and granulated sugar at medium-low speed until smooth and creamy. Mix in sour cream and vanilla extract until fully combined. Add the eggs one at a time, blending gently to maintain texture. Carefully pour the cheesecake filling over the cooled brownie, ensuring even distribution, then prepare for baking.

Place the springform pan in a larger baking dish filled halfway with hot water to create a water bath, helping the cheesecake bake evenly. Bake the Hot Fudge Sundae Brownie Cheesecake for 50-60 minutes until the edges are set, but the center should still jiggle slightly when moved. Turn off the oven, crack the door, and let it cool inside for an hour.

Heat heavy cream in a small saucepan over medium heat until it simmers, then remove from heat. Pour the hot cream over finely chopped dark chocolate in a bowl. Let it sit for a couple of minutes before whisking until completely smooth and glossy. Allow the fudge sauce to cool at room temperature until it reaches a pourable consistency.

Once the cheesecake has set in the fridge, carefully release it from the springform pan and transfer it to a serving platter. Generously pour the cooled fudge sauce over the top, letting it cascade down the sides. For a fun finishing touch, pipe whipped cream swirls onto the cheesecake and crown it with maraschino cherries and chocolate shavings.