Combine cake mix, butter, cocoa powder, eggs, and vanilla in a large mixing bowl and mix by hand until blended.

Fold in white chocolate chips. Refrigerate for 30 minutes.

Preheat oven to 375 degrees.

In a medium sized bowl, combine powdered sugar and cornstarch. Mix with a fork.

Line cookie sheets with parchment paper.

Form 1 inch balls with cookie dough and roll in powdered sugar mixture.

Place about two inches apart on cookie sheet.

Bake for 9-11 minutes. Do not overbake!

Let cool on cookie sheet for 2-3 minutes before transferring to a cooling rack.

Store in an airtight container for up to 1 week.