Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper; set aside.

In a large bowl, combine the cake mix, oil OR melted butter, and eggs and stir with a rubber spatula until a soft cookie-dough-like consistency forms and comes together, scraping down the sides and bottom of the bowl as needed to incorporate any dry pockets. Stir in your mix-ins of choice, or if you want several mix-ins, divide the dough among a few bowls and mix in a couple Tablespoons of each mix-in to each bowl (for instance, a few Tablespoons of raisins to one bowl, a few Tablespoons of chocolate chips to another, etc).

Using a 1-Tablespoon-size cookie dough scoop, portion out cookie dough balls onto the baking sheets about 2" apart. Bake for 8-10 minutes, rotating the pans halfway through baking time to ensure even cooking. Remove from the heat and allow to cool for about 10 minutes on the sheets before transferring to wire racks to cool completely. Store any leftover cookies airtight at room temperature.