Preheat your oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners. In a medium bowl, whisk together the melted butter, sugar, and cocoa powder. Add the egg and vanilla and mix until combined. Stir in the flour and salt until just incorporated.

Spoon approximately 1.5 tablespoons of batter into each liner. Bake for 10-12 minutes until the brownie layer is just set.

While the brownies bake, beat the softened cream cheese and sugar with an electric mixer until smooth. Add the egg, sour cream, and vanilla, mixing on low speed until just combined. Do not overmix.

Remove the muffin tin from the oven and immediately spoon the cheesecake filling over the hot brownie layer, filling each liner.

Return the tin to the oven and bake for an additional 15-18 minutes, until the edges are set and the center has a slight wobble.

Let the mini cheesecakes cool completely in the muffin tin at room temperature. Then, refrigerate for at least 4 hours or overnight before serving.