For the cake mix, put flours into the bowl of a food processor.

Add sugar, baking powder and salt and process until blended.

Cut butter into 4 or 5 chunks, then add to flour mixture and pulse until completely incorporated.

Use cake mix immediately (see below) or store in the refrigerator in an airtight container for up to 1 month.

For the yellow cake, preheat the oven to 350°F.

Butter and flour a 9x13-inch cake pan or two 8-inch round cake pans; set aside.

Transfer cake mix to large mixing bowl, then add eggs, milk and vanilla and beat with an electric mixer on low speed for 1 minute.

Increase speed to medium high and beat for 2 minutes more, scraping sides of bowl occasionally.

Transfer batter to prepared baking pans.

For a 9x13-inch cake, bake for 40 to 50 minutes, or until done when tested with a toothpick.

For two 8-inch cakes, baking time may shorten, so check after 25 to 35 minutes.

Set cakes aside to cool for about 10 minutes, then remove them from the pans and set aside on wire racks to cool completely. Frost cakes, if desired.