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.