In a large bowl, add yellow cake mix, eggs, vegetable oil and vanilla extract. Mix everything together, then fold in any mix ins you want such as 1 cup chocolate chips, 1/2 c. chopped nuts.
Leave in bowl, cover with plastic wrap and refrigerate ideally for 30-60 minutes to stiffen up so they don't spread as much. Preheat the oven to 350 degrees F.
Scoop tbsp. of batter onto prepared baking sheets with space in between. Bake for approximately 8-10 minutes, or until edges are firmed and middle is no longer wet. Timing depends on how large of cookies you make. Remove from the oven, leave on sheet for 5 minutes and then move to cooling rack.
Tip: As soon as the edge all the way around are firming up and cracking + middle is not yet firm but not sticky and liquid either you want to remove from the oven. In that 5 minutes cooling on the baking sheet out of the oven they will continue to solidify enough to transfer to cooling rack with a spatula.