I'm not knocking substitutions. There are loads that do work. There's even a bible on the topic. But watch out for the fat-free dairy products; they often contain stabilizers or thickeners that can wreak havoc with your baking. Ditto for some of the butter substitutes. And for goodness sake, don't substitute Cool Whip for whipped cream in anything baked or cooked.
Chocolate poses other pitfalls. When a recipe calls for bittersweet chocolate, control your super-charged urge to use extra-bittersweet versions in the 70 and 80 percent cacao range. The higher the percentage, the less sugar in the bar, and that lack of sugar could seriously affect the texture of your baked good. Stick with the 60 percent range for bittersweet if the cacao percentage isn't listed in the recipe. And if bittersweet bars you see don't state the cacao content, chose one from from a well-respected chocolate company. Chances are, it will have slightly less than 60 percent, but way more than the legal minimum of 35 percent.
The worst story I ever heard? A cook used canned clams in place of puréed raw scallops in a terrine (what was she thinking?), and wondered why it was still liquid after baking!
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