Line an 8-inch square cake pan with parchment paper and preheat the oven to 350°F / 180°C (160°C fan assisted).
Put the white sugar and light brown sugar in a large mixing bowl, then add the melted butter and whisk together until combined.
Add the eggs, egg yolk and vanilla and mix together. Be careful not to overmix - we are not trying to add air into the mixture as that will make the brownies cakey.
Add the vegetable oil and vinegar to the brownie batter and whisk until it looks glossy and smooth.
Take a couple of tablespoons of the batter and put them into a small bowl. Add your red gel food coloring to this bowl and mix it in until the gel is all incorporated and there are no lumps. Add this mixture into the main bowl of batter and mix until combined.
Sift the dry ingredients and mix them together, then fold them gently into the brownie batter in two batches, using a rubber spatula or wooden spoon. Be very careful not to overmix here as it can make the brownies dense or cakey.
Optional: If you like, you can add some white chocolate chips at this stage. I decided I liked the brownies better without.
Pour the brownie batter into the prepared cake pan and bake until the edges are set and pulling away from the pan, but the centre still looks slightly soft. Baking time will vary depending on your oven and pan, but mine takes roughly 20-25 minutes. If you insert a cake tester in the middle, it should come out with either moist crumbs or a very thin layer of batter. The brownies will continue to bake as they cool.
Remove from the oven and let cool completely before frosting and slicing.
Beat the room temperature butter and cream cheese together until smooth and combined.
Add the vanilla and powdered sugar and mix in slowly to avoid a dust cloud. You may need more or less powdered sugar depending on the thickness of your cream cheese, so add it 1 cup at a time until it is thick enough to spread. Beat for a few minutes until light and fluffy.
Spread over the red velvet brownies before slicing.
Notes
Ingredient Notes:
Cocoa Powder - Use dutch-processed cocoa powder for brownies that are more fudgy than cakey and have a subtle cocoa flavour that won't overpower the other ingredients. Eggs - Use large eggs in the US or medium eggs in the UK (our eggs are bigger). Cream Cheese - Cream cheese is one of those ingredients that is wildly different depending on where you are in the world. US cream cheese is a lot firmer than UK cream cheese, so you'll need a lot less powdered sugar to turn it into frosting. The best thing to do is add the powdered sugar gradually until your cream cheese frosting is thick enough to hold soft peaks. I have a post all about the difference between US and UK baking ingredients if you'd like to learn more. I recommend with all my recipes that it's best to weigh ingredients with a scale for the most accurate measurements and most consistent results. With this recipe, it is especially important that the measurements are accurate, as the tiniest tweak in ingredients can hugely impact the texture of the brownies. Just a little too much flour for example, can make the brownies have more of a cake-like texture than a chewy brownie texture - so if you can, use a scale!