When it comes to recipes, it is usually best to match the ingredient list as closely as possible. Use large eggs, when a recipe calls for them. Use buttermilk, rather than a substitute if you can. You’ll usually get the best flavor and the most accurate results this way.
The biggest temptation – these days, it seems – is to substitute lower fat ingredients in place of their full-fat counterparts in the name of health. Low fat or skim milk in place of whole milk doesn’t have that much of an impact on a finished cake, since it really only reduces that fat content slightly. Most of the fat comes from butter (or oil) and products like “light butter” and “whipped butter,” both of which appeal to consumers as spreads for toast and bread, seem like tempting substitutes for the real deal.
The problem with whipped and light butters is that they’re not going to give you the results you want. Real butter is 80% milk fat and 20% water. This ratio is crucial to producing tender cakes and cookies, as well as flaky pie crusts and pastries. Whipped butter is made by creating air pockets in standard butter (this makes it easier to spread) and can be up to 1/3 air. If you try and substitute this product, you’ll end up with far less butter than you should – and a less tender, less buttery product. “Light” butter generally has more water and/or fillers added to it to replace butterfat. This is a poor substitute for real butter because recipes don’t account for fillers and the extra water (and other ingredients, for that matter) is not going to contribute to a perfectly textured product.
Use the real thing and take smaller bites, if you’re wanting to cut back on the fat content in some of your baked goods, or look up a recipe that is lower in fat to begin with. If you want to try the substitutes, be aware that they’re not really going to be comparable to the same recipe made with real, full-fat butter.