Preheat the oven to 350°F/180°C and line 2 large cookie sheets with parchment paper or Silpat silicone mats.
In a small bowl, combine the granulated sugar with cinnamon and set aside.
In a large bowl, combine cake mix with melted butter, egg, and vanilla. Mix with a hand mixer or just a spatula until well combined (the mixture will be very thick and sticky).
Using a medium OXO cookie scoop (1.5 inches in size), scoop out cookie dough balls, then roll in the cinnamon sugar and place them on the prepared cookie sheets about 2 inches apart.
Bake in a preheated oven for 10-12 minutes, then cool down completely on a wire rack.
Notes
Smaller Cookies: If you're adding these to a cookie tray, you may want to make them smaller. just use a small cookie scoop (1-inch size = 1 tablespoon), and decrease the baking time by 1-2 minutes. You will get 38 cookies in total.
Be sure to allow 2 inches of space around each cookie when baking. These cookies spread a little, so you want to be careful not to put them too close on the cookie sheet.
Avoid getting flat cookies: Some cake mixes (especially brands outside the US) can make flat cookies. The best way to tell if the cookies are going to be flat is with the consistency of the cookie dough, if it’s way too sticky and too soft then you can add about 1/4 to 1/2 cup of flour to the mixture. I used Betty Crocker cake mix and did not need to add any flour to the dough.
Storing: Store these cake mix cookies in a single layer in an airtight container and enjoy them within 2-3 days. The cookies can be kept longer, but they will start to get stale after 3 days and won’t be as soft and delicious.