Add all ingredients, except the butter, into a food processor.
Process for 15 seconds, until combined.
Add the butter and lightly fluff with a fork to coat with the flour mixture.
Add vanilla, then pulse until the mixture is crumbly and fine.
Use immediately in any recipe calling for yellow cake mix, or freeze immediately in a zipper bag for up to 2 months.
If not using right away, transfer to a zipper bag and freeze immediately.
If you allow the mixture to sit out too long, the butter will start melting and change the texture of the dry mix.
If this does happen, freeze it for about 1 hour then throw it back in the food processor and pulse until it reaches the fine consistency.