• Preheat your oven to 350°F/180°C. Put parchment paper in an 8×8-inch baking pan.

Make the brownie batter:

• In a big bowl that can handle heat, put butter, cocoa powder, and salt. Heat it in the microwave for 30 seconds at a time. Stir each time until it's all melted.
• Next, add sugar and whisk it until everything is mixed together. After that, add the eggs, vanilla extract, and vinegar. Keep whisking until it's all mixed up. If you want a specific color, add red food coloring until you like how it looks. Then, add flour and gently stir with a rubber spatula until it's just mixed.
• Take ⅓ cup of the batter and set it aside. Pour the rest into the pan you prepared and make the top smooth.

Make the cheesecake batter:

• Use a mixer or a hand whisk to blend the cream cheese and sugar until it's smooth (medium speed if you're using a mixer). Make sure to scrape the sides and bottom of the bowl as you go. Add the egg and vanilla extract, and keep beating until everything is mixed.
• Now, spoon the cheesecake mixture in dollops over the brownie batter. Take the ⅓ cup of reserved brownie batter and also spoon dollops over the mixture. Use a knife or skewer to gently swirl the two batters together to create a marble effect.
• Bake in the oven for around 25-30 minutes. You'll know it's done when the cheesecake part is set, slightly puffed, and a toothpick inserted into the brownie part comes out with moist crumbs (not dry). Let the brownies cool completely, then refrigerate for at least 2 hours until they're firm enough to cut.
• Once they're firm, lift them out of the pan using the parchment paper and cut them into squares. The brownies actually taste even better the day after you make them.
• If you want to store them, keep the brownies in the fridge in an airtight container. They'll stay good for up to 5 days.