Preheat your oven to 350°F (175°C). In a medium saucepan, melt together unsalted butter and semisweet chocolate over low heat, stirring until smooth. Remove from heat and allow to cool slightly. Mix in granulated sugar, followed by eggs one at a time, then stir in vanilla extract, all-purpose flour, and salt until combined.

Line a mini muffin pan with paper liners. Pour the brownie mixture into each cavity, filling them about one-third full. Bake for approximately 8 minutes, until edges look set but centers are still soft. Remove from oven and cool slightly.

In a separate mixing bowl, beat softened cream cheese until smooth. Gradually add granulated sugar and vanilla, mixing well. Incorporate an egg and sour cream into the mixture, beating until just combined.

Spoon cheesecake batter over cooled brownie layers, filling nearly to the top. Lower oven temperature to 325°F (163°C) and bake the mini cheesecakes for 15 to 18 minutes.

Let mini cheesecakes cool in the pan for about 30 minutes. Then carefully transfer to the refrigerator and chill for at least 2 hours.

Once chilled, take mini cheesecakes out of the fridge. Add desired toppings and serve.