Preheat the oven to 375 degrees F. Line two baking sheets with parchment paper.
In a large bowl with an electric mixer, beat the butter until slightly creamy, about 1 minute.
Add the sugar and beat another minute, until well combined.
Add the eggs and egg yolk, one at a time, scraping down the edges between each addition.
Add the vanilla, and beat to combine.
Add the baking powder, baking soda, and salt, and beat to combine.
Add the cake mix and flour, beating until combined.
Spray a ⅓ cup measuring cup with cooking spray (this keeps the dough from sticking) and form the dough into ⅓ cup balls.
Place 3 dough balls on each baking sheet (you will need to do this in two batches).
Bake for 9-10 minutes in preheated oven. The edges should be slightly golden and the middle set. Cool on the baking sheet for 5 minutes and then transfer to a cooling rack to cool COMPLETELY.