Preheat your oven to 350°F (175°C). Lightly spray a 9-inch square baking pan and line it with parchment paper, leaving some overhang on the sides – this makes lifting them out super easy!
In a large, microwave-safe bowl, combine the 12 ounces of chocolate chips and cubed butter. Heat in the microwave for 30-second bursts, stirring well after each, until completely smooth and melted.
Stir the ¾ cup sugar and 1 teaspoon vanilla extract into the melted chocolate mixture until fully combined. Beat in the 3 eggs, one at a time, making sure each is well incorporated before adding the next.
In a separate small bowl, whisk together the cocoa powder, cornstarch, and salt.
Add the dry mixture to the wet chocolate mixture. Stir vigorously for 1 to 2 minutes until the batter is smooth, thick, and starts to pull away from the sides of the bowl. Then, gently fold in the 1 cup of mini chocolate chips.
Pour about two-thirds of this amazing brownie batter into your prepared pan and spread it evenly with a spatula. Set the remaining batter aside for later.
In a stand mixer (or with a hand mixer), beat the softened cream cheese until it's light and fluffy. Add the ½ cup sugar and beat until just combined. Finally, beat in the remaining 1 egg and ½ teaspoon vanilla extract until the mixture is smooth.
Carefully spoon the creamy cheesecake filling over the brownie batter in the pan. Gently spread it out into a thin, even layer with an offset spatula.
Drizzle or spoon the reserved brownie batter over the cheesecake layer. For a pretty marbled effect, you can gently swirl it with a knife.
Bake for 30 to 35 minutes. You'll know it's ready when a toothpick inserted into the center comes out with just a few moist crumbs. Keep an eye on them after 30 minutes – ovens can vary!
Let the brownies cool completely in the pan on a wire rack. Once cool, use the parchment paper overhang to lift them out. Cut into squares and store any leftovers in an airtight container in the refrigerator. Enjoy!