For about 4 cups:
4 cups flour
8 tsp baking powder
1 tsp salt
Alison Holst’s Baking Mix seems to be unavailble at all supermarkets. Here is an equivalent recipe that is easy to make at home.
Measure the dry ingredients into a bowl or food processor. Add the butter and cut in until it the mixture resembles very fine crumbs.
Transfer to an airtight container and refrigerate. Best used with 2-3 weeks.