In very large bowl, combine butter, salt and baking powder; blend well. (Lightly spoon flour into meassuring cup; level off. Do not scoop or sift flour.) Add flour to butter mixture; blend until fine crumbs form, scraping bowl often. Use in following cookie recipes. Measure by dipping cup into mix; level off. Store tightly covered in refrigerator up to 4 weeks. Makes 8 cups of mix.