Ghirardelli, Premium Baking Bar, Unsweetened Chocolate, 4 oz (113 g) - iHerb.com
Ghirardelli 100% - 1 ingredient: Unsweetened chocolate. 0g sugars. $2.77 for 4 ounce bar.
I keep it at my desk at work. I only eat 1/4 of a square at a time; it's bitter but not terribly so.
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I started using real cocao butter with just a little bit of coconut oil to have the chocolate stay firm outside of the fridge, just like the storebought chocolates. Just substitute the coconut oil with cocoa butter one on one.