Preheat your oven to 350°F (175°C). Line a 12-cup standard muffin tin with paper liners.

In a medium bowl, whisk together the melted butter and sugar until combined. Add in the egg and vanilla extract, stirring until smooth. Gradually mix in the flour, cocoa powder, and salt until just combined.

Spoon about 1 tablespoon of brownie batter into each liner and press it down into an even layer. Bake for 10-12 minutes.

In a large bowl, beat the softened cream cheese until smooth. Gradually add the sugar and mix again until combined. Then mix in the egg, sour cream, and vanilla on low speed until everything is just combined.

Once the brownie bases are baked, spoon the cheesecake filling over the warm brownie bottoms.

Bake for an additional 15-18 minutes, or until the edges are set and the center has a slight jiggle.

Turn off the oven and let the cheesecakes cool inside with the door ajar for 1 hour. Afterward, let them cool completely at room temperature.

Once cooled, chill them in the refrigerator for at least 4 hours before serving.