Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease an 8-inch square baking pan, lining it with parchment paper.
In a medium bowl, whisk together the cream cheese, sugar, 1/2 egg, and vanilla. If you have a carton of pre-beaten pasteurized eggs on hand, add 1 1/2 tablespoons in place of the 1/2 egg. Set this mixture aside.
To a different mixing bowl, add the melted butter with the cocoa powder and whisk together until smooth.
Whisk in the sugar, then the eggs, then the vanilla, gel food colouring, and vinegar. Whisk the mixture together well until smooth.
Add the flour and salt and mix together with a spatula just until the flour disappears and everything is combined.
Add the thick batter to the prepared pan and press the batter to the edges and into the corners of the pan with the back of the spatula or with an offset spatula.
Gently score the top of the unbaked brownie batter in the pan to create a tic-tac-toe pattern (dividing it into 9 equally-sized squares).
In the centre of each square, add one dollop of the cheesecake mixture (about 1 1/2 tablespoons) into the middle. If you're creating the heart design, you may have some leftover cheesecake mixture to discard.
Tap the pan firmly against the table to settle the batter and the cheesecake mixture.
Take a butter knife or a dinner knife (not a sharp knife) and drag the knife (held vertically) through each of the cheesecake dollops to create the hearts (watch the video in this post for a tutorial).
Bake for 22-25 minutes at 350 degrees Fahrenheit (175 degrees Celsius), turning the pan once about halfway through baking to ensure the cheesecake hearts cook evenly and don't over-brown on one side.
Let the brownies cool for about 20-30 minutes before slicing and serving.