Preheat the oven to 350 degrees Fahrenheit and line cookie sheets with nonstick baking mats or lightly spray with nonstick baking spray.

In the bowl of an electric mixer, mix together cake mix, egg, vanilla, softened butter and sour cream.

Scoop or form cookie dough into 1 1/2" balls, then roll the balls in Halloween sprinkles.

Place dough balls at least 2" apart on the prepared sheets.

Use the bottom of a completely flat drinking cup or measuring cup to press dough balls down, as shown in the image above.

Bake for no more than 7-8 minutes. Allow cookies to cool on the cookie sheets for a couple minutes before carefully removing them to a cooling rack to cool completely. After they're cooled completely, store them in an airtight container.