01 - Preheat the oven to 350°F. Line a 24-cup mini muffin tin with paper liners or lightly grease each cup.
02 - In a medium bowl, whisk together melted butter and sugar until smooth. Add the egg, vanilla extract, and red food coloring; mix until combined.
03 - Sift in the flour, cocoa powder, and salt. Stir until just incorporated—do not overmix.
04 - In a small bowl, beat together the softened cream cheese, sugar, egg yolk, and vanilla until smooth and creamy.
05 - Spoon about 1 tablespoon of brownie batter into each muffin cup. Add 1 teaspoon of cream cheese mixture on top of each.
06 - Use a toothpick or skewer to gently swirl the cream cheese into the brownie batter for a marbled effect.
07 - Bake for 23–25 minutes, or until the centers are set and a toothpick inserted comes out with a few moist crumbs.
08 - Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.