I bought a two-pound bar of bittersweet chocolate, instead of unsweetened. Is it interchangeable with unsweetened for baking a cake? If so, how? Thank you.
Unsweetened chocolate is 99% cacao. The rest is vanilla nad emulsifiers. Semi-sweet or bittersweet are usually around 60% cacao, 40% sugar.
You can compensate by increasing the total amount of chocolate, and reducing the sugar you add accordingly.
For example, if your recipe calls for 6 oz of unsweetened choc., you would use 10 oz of bittersweet, and reduce the sugar by 4 oz.
My suggestion would be to save the bittersweet for something else and go buy unsweetened chocolate for your cake.
If you do try to substitute the bittersweet, just pay attention to its actual composition, since the percentage of cacao solids can vary wildly among products labeled "bittersweet." In addition to adjustments described by PDXpat, you'll need to adjust the level of fat in the recipe as well. (By adding more chocolate, you're also adding more cocoa butter--or vegetable fat, if it's a cheap chocolate.)
I don't believe it's quite that simple. Assuming you know that your bittersweet chocolate is 60% (which will rarely be the case, if you're using typical supermarket chocolates), the 60% would be of cacao solids--i.e., cacao mass/liquor, cocoa, and/or cocoa butter. The remaining 40% would be everything else, including sugar, milk solids, vegetable fats, and/or small amounts of vanilla (or vanillin) and soy lecithin. So you typically have fat components on both sides of the percentage (i.e., added cocoa butter and any naturally occurring cocoa butter in cacao mass on the cacao solids side, and milk fat and/or vegetable fats on the other side).
If we were to assume that: (a) the unsweetened chocolate in question is roughly 100% cacao solids (which will never be the case with a supermarket chocolate); (b) the bittersweet chocolate's cacao solid composition is exactly proportionate with that of the unsweetened chocolate (which will rarely be the case); and (c) the bittersweet chocolate's non-cacao solid portion contains no milk fat (which will almost never be the case with a supermarket chocolate), then your formula would be correct. I just don't think that will ever be the case (and, if it were, it would be difficult to know). Often, the 10 ounces of bittersweet chocolate in your example will contain more total fat than the 6 ounces of unsweetened. (Though they don't provide the reasoning behind their answer, you can get the thrust of it from the advice on substitutions on the "Chocolate How To's" page at Scharffen Berger's web site: http://www.scharffenberger.com/howto.... .)
Most cake recipes for home cooks are geared towards supermarket chocolate brands (e.g., Baker's, Nestle, etc.). With these supermarket unsweetened chocolates, you can't be sure what you're getting. The ingredient list for Baker's Unsweetened is "chocolate, milk solids." What percentage of the product is milk solids? Your guess is as good as mine. And what exactly is "chocolate"? The FDA defines it expansively in 21 CFR 163.111, allowing it to include a variety of non-cacao-derived additives (including "butter or milkfat"). There are a lot of wild cards on the fat side, not to mention the unknown breakdown of cacao solids versus everything else.
I get the impression that you're talking about finer chocolates, which will usually specify the percentage of cacao solids, with the remainder being almost entirely sugar. That doesn't really apply to the common supermarket chocolates that are presupposed in most recipes for home cooks.
Like I said to begin with, I'd just save the bittersweet for something else and go buy some unsweetened chocolate.
I do it all the time. Here in the UK I have a hard time finding unsweetened baking chocolate. I always use bittersweet, and reduce the sugar slightly. Actually, I eyeball it, thinking "hmm, how much sugar could be in that chocolate bar?", then reduce by a couple of spoonfuls. Very scientific, I know. But I've never known any of my cakes or brownies or cookies to suffer because of it.