Mix all dry ingredients in a bowl and sift three times to make sure they are well-blended.

Cut shortening or cold (not softened or melted) butter with a pastry blender until mixture resembles fine crumbs. (If you have a large enough food processor, that'll work too.)

Store mixture in an airtight container or ziplock bag for up to four months. Note that if you use butter, you will need to store this in the refrigerator, to keep the butter from melting and/or going rancid.