Shortenings (solid vegetable shortening, butter and margarine) give cookies tenderness and moistness. Butter offers more flavor and usually gives a crisper cookie, but the cookie and bar recipes in this chapter will all yield excellent results when made with regular butter or regular margarine.
For best results, do not use whipped margarines or butters and low-fat spreads. Their behavior in baking is unpredictable because they contain a higher proportion of water and air to make them more spreadable.
Vegetable oil does not allow the dough to incorporate air during beating. Do not use it as a substitution when the cookie recipe calls for solid shortening.