Preheat oven to 350°F (175°C) and grease a 24-cup mini muffin tin or use paper liners.
In a medium bowl, combine melted butter, 1 cup sugar, cocoa powder, and 1/2 teaspoon vanilla extract.
Whisk in 2 eggs until the mixture is smooth and glossy.
Stir in the flour and salt until just combined; do not overmix.
In a separate bowl, beat the softened cream cheese, 1/4 cup sugar, remaining 1/2 teaspoon vanilla, and the final egg until smooth.
Fill each mini muffin cup approximately 2/3 full with the brownie batter.
Place a small teaspoon of the cheesecake mixture on top of each brownie-filled cup.
Using a toothpick, gently swirl the cheesecake mixture into the brownie batter to create a marble pattern.
Bake for 18 to 22 minutes until the edges are set and a toothpick inserted into the brownie comes out with few moist crumbs.
Cool completely in the tin before removing to allow the cheesecake layer to firm up.