1. Prepare Oven and Pan:

• Preheat your oven to 350°F (175°C).
• Lightly spray a 9-inch square baking pan with cooking spray. Line the pan with parchment paper, ensuring there’s an overhang on at least two opposite sides (this will help you lift the brownies out later). Lightly spray the parchment as well. Set aside.

2. Make the Brownie Batter:

• Melt Chocolate & Butter: In a large microwave-safe bowl, combine the 12 ounces of semi-sweet or bittersweet chocolate chips and the 6 tablespoons of cubed unsalted butter. Microwave in 30-second increments on high heat, stirring well after each interval, until the chocolate and butter are completely melted and the mixture is smooth. Be careful not to scorch the chocolate.
• Add Sugar & Vanilla: Stir the ¾ cup granulated sugar and 1 teaspoon vanilla extract into the melted chocolate mixture until completely combined and smooth.
• Add Eggs: Add the 3 room temperature eggs one at a time, stirring well with a whisk or spatula after each addition until just combined.
• Combine Dry Ingredients: In a separate small bowl, whisk together the ¼ cup unsweetened cocoa powder, ⅓ cup cornstarch, ½ teaspoon salt, and ¼ teaspoon baking powder. Sifting these helps prevent lumps.
• Finish Brownie Batter: Add this dry mixture to the wet chocolate mixture. Stir vigorously with a spatula until the batter is smooth, glossy, and begins to pull away slightly from the sides of the bowl. This might take one or two minutes of firm stirring.
• Gently stir in the 1 cup of mini semi-sweet chocolate chips.

3. Prepare for Layering:

• Pour about two-thirds (⅔) of the prepared brownie batter into the parchment-lined 9-inch square pan. Use an offset spatula or the back of a spoon to spread it into an even layer.
• Set aside the remaining one-third (⅓) of the brownie batter.

4. Make the Cheesecake Layer:

• Ensure the cream cheese is fully softened and the egg is at room temperature.
• In a medium mixing bowl, combine the softened cream cheese and 1/2 cup granulated sugar. Beat with an electric mixer (handheld or stand mixer with paddle) on medium speed until smooth and creamy, with no lumps.
• Add the room temperature egg and ½ teaspoon vanilla extract. Beat again on low speed just until combined and smooth. Do not overmix.

5. Assemble and Swirl (or Layer):

• Dollop spoonfuls of the prepared cheesecake mixture randomly over the brownie batter in the pan.
• Add dollops of the reserved one-third (⅓) of the brownie batter in the gaps between the cheesecake dollops.
• Optional Swirling: Use a knife or a skewer to gently swirl the two batters together to create a marbled effect. Don’t over-swirl, or the colors will become muddy. (Alternatively, for distinct layers, spread all cheesecake batter over bottom brownie layer, then carefully spread remaining brownie batter on top, though swirling is more common for this style).

6. Bake:

• Place the pan in the preheated oven.
• Bake for 30-35 minutes, or until the edges are set and a toothpick inserted into a brownie-only portion (avoiding cheesecake swirls) near the center comes out with a few moist crumbs attached. The cheesecake portions may still look slightly soft but should be mostly set.

7. Cool Completely and Chill:

• Remove the pan from the oven and place it on a wire cooling rack.
• Let the cheesecake brownies cool completely in the pan at room temperature (this can take 2-3 hours).
• Once completely cool, cover the pan with plastic wrap and transfer to the refrigerator. Chill for at least 2-3 hours, or preferably overnight, until thoroughly cold and firm. Chilling is crucial for clean slicing and the best texture.

8. Slice and Serve:

• Once thoroughly chilled, use the parchment paper overhangs to carefully lift the entire block of brownies out of the pan onto a cutting board.
• Use a large, sharp knife to cut the block into squares or bars. For the cleanest cuts, dip the knife in hot water and wipe it clean with a paper towel after each cut.
• Serve the Cheesecake Brownies chilled or at cool room temperature. Store leftovers tightly covered in the refrigerator.