Add all ingredients except for butter and vanilla to the bowl of a large food processor. Pulse for about 15 seconds, just to combine. Add the butter and vanilla and process until the mixture resembles a coarse meal – you want it to look much like a boxed mix would.
Store the mixture in a zip-top bag in the freezer for up to two months.
Preheat oven to 350°F. Grease and flour the pan of your choice (see recipe yield).
Use an electric mixer to beat together the prepared mix, 1 1/4 cups warm water and 2 large eggs until smooth, about 2 minutes.
Pour into the prepared pans and bake:
about 25 minutes for 9-inch layer cakes
30-35 minutes for a 13x9-inch cake
45 minutes for a bundt cake
18-20 minutes for cupcakes.
Notes
I recommend Nielsen Massey Vanilla Extract
To use the mix in a recipe calling for 1 18-ounce box of cake mix, use about half, or about 3 1/2 cups, of the prepared mix.