Cheesecake Filling: Preheat the oven to 325°F (160°C). Line an 8-inch (20 × 20 cm) square pan with parchment paper.
In a separate bowl, whisk together the cream cheese, sugar, cornstarch, egg, and vanilla extract until smooth and creamy. Set aside while you prepare the brownie batter.
Red Velvet Brownie: In a bowl, sift together the all-purpose flour and cornstarch. Set aside.
Microwave the chocolate in a large bowl at 50% power, stirring occasionally, until melted, about 2 minutes. Whisk in the cocoa powder until fully incorporated, then let the mixture cool slightly.
Add the granulated sugar, avocado oil, white vinegar, vanilla extract, and red food coloring, and mix until the batter looks smooth and glossy.
Beat in the eggs, mixing on high speed with an electric mixer for about 1–2 minutes until slightly thickened.
Add the dry ingredients to the batter and mix on low speed just until combined. Pour most of the brownie batter into the prepared pan, reserving about 4 teaspoons for the swirl topping. Spread the batter evenly.
Spoon the cheesecake filling over the brownie layer and gently spread it. Drop small spoonfuls of the reserved brownie batter on top, then use a toothpick or knife to create a swirl pattern.
Bake for 50–55 minutes. Allow the brownies to cool in the pan for 30 minutes, then refrigerate for at least 1 hour before slicing and serving.