Line a 9x13 baking dish with foil. Spray the foil with non-stick cooking spray and set it aside.
Prepare Brownie Mix
In a large bowl, prepare brownie mix according to package instructions.
Spread onto the bottom of the 9x13 pan in an even layer. Set aside.
Prepare Red Velvet Cake Mix
Combine red velvet cake mix, oil, egg, and milk. MMix until completely incorporated. The batter will be thick.
Spoon red velvet batter into the brownie batter layer and swirl very gently with a butter knife in different directions.
Bake for 22-28 minutes or until a toothpick comes out almost clean with a small amount of batter on it.
Allow red velvet brownies to cool completely before cutting into them.
Notes
You should prepare brownie mix according to the directions on the back of the box.
I have included the ingredients that my favorite boxed brownie mix requires. If you are using a different brand, please use the measurements listed on the box instead of what I have listed here.