Preheat your oven to 350°F (175°C).
Melt the unsalted butter in a saucepan over medium heat.
In a mixing bowl, combine melted butter and sugar, stirring until well blended.
Add the eggs one at a time, mixing well after each addition.
Mix in flour, cocoa powder, and vanilla extract until just combined.
Prepare your cupcake pan with liners or grease with non-stick spray.
Pour brownie batter evenly into the prepared cupcake pan, filling each cup about 2/3 full.
Bake the brownie layer for 15-18 minutes until the edges begin to set and a toothpick comes out with a few moist crumbs.
While the brownies cool, prepare the cheesecake filling by beating the softened cream cheese with sour cream and sugar until smooth.
Add eggs to the cheesecake mixture one at a time, mixing well after each addition.
Fold in mini chocolate chips, if using.
Spoon the cheesecake filling over the cooled brownie layer, distributing evenly.
Bake everything together for an additional 20-25 minutes or until the cheesecake is set in the center.
Allow to cool before refrigerating for at least 2 hours.
Top with whipped cream, sprinkles, or crushed nuts just before serving.