Preheat the oven to 375 degrees F. Line 2 baking sheets with parchment paper.
In a large bowl with an electric mixer, beat the butter until combined, about 1 minute.
Add the sugar, and beat until combined.
Add the eggs, one at a time, scraping down the edges after each addition.
Add the egg yolk, beating until combined.
Add the baking powder, baking soda, and salt, and mix well to combine.
Add the vanilla extract, beating until combined.
Add the red velvet cake mix, mixing until combined, followed by the flour. Be sure to scrape down the edges as needed to ensure it is all combined.
Using ½ cup measuring cup, form the dough into ½ cup balls. This will make 6 cookies.
Place 3 cookie balls on each prepared sheet and press them down to slightly flatten them.
Bake one sheet at a time in the preheated oven for 9-10 minutes. They may seem underdone in the middle, but that is okay! They will set up completely as they cool.
Repeat with second baking sheet.
Cool for 10-15 minutes on the baking sheet and then carefully transfer to cooling rack to cool completely.