This can be made in 3 batches in a large food processor.
Whiz regular oatmeal, measure 1 cup, whiz in the nutritional yeast. Mix thoroughly with cake flour or substitute, then carefully mix in salt, baking soda, baking powder, and sugar. Blend this with the buttermilk powder until it is uniform- a sifter is handy here if you aren't using a food processor.
In the food processor or a large bowl, cut together 1/3 of the flour mix, 1/3 each of the shortening and butter until it resembles coarse cornmeal. Set aside and repeat until all is used, then mix the three batches together before you divide it for storage or use. Store in tightly closed covered container or zippered plastic bags with the air pressed out.
Even with the added flours, this makes a golden white biscuit unless whole wheat flour is added.
To use, spoon into cup, don't sift and don't pack. Store remainder in refrigerator or freezer.
Reduced fat variation: you can reduce the shortening in this mix by 1/4, results will be a bit less tender, but still tasty. With reduced fat baking recipes, smaller sizes muffins, biscuits and pancakes will give better results. Fat may be reduced in most baking recipes by one-third. Fat makes baked goods tender and crisp, so lowering the amount of fat will change the texture as well as the flavor. With pie crusts, the fat may be decreased to one part fat to four parts flour. Some have had success also with replacing some of the fat in quick bread recipes with the same amount of applesauce. I have found it does work but the texture is tougher than the original recipe.