If you've ever tempered chocolate before (hopefully while consulting Kenji's excellent guide), you've likely had a good bit left over—whether you're dipping or coating candies, fruit, and cookies, it takes some extra chocolate to ensure you've got enough wiggle room to get the job done.
Now, an extra ounce or two of leftover chocolate is easily managed with a guilty glance and a quick finger to the bowl, but if you've got more than that, it can be saved and reused for another project. Not for tempering, as it's likely tainted with cookie crumbs, swirls of butter, or traces of caramel, but for baking, where such contaminants are both tasty and harmless.
Whatever you do, don't let that leftover chocolate harden in the bowl; it'll be a pain to remove. And don't transfer it to any sort of plastic container, as it may solidify into a puck too thick to chop easily. Instead, as soon as you've finished using it, pour that chocolate straight onto a sheet of parchment, and pop it in the fridge until it's hardened through and through.
If you've used that chocolate for dipping cookies or caramels, you may notice some unsightly but harmless streaks or swirls of grease in it. They're not pretty, but assuming your desserts aren't infused with the intensity of liquid smoke or a pure essential oil, those trace amounts of fat won't contribute any discernible flavor to the chocolate.
Unless you managed to stir and scrape with the efficiency of a machine throughout the dipping process, you'll probably notice a grayish bloom over the leftover chocolate—maybe not immediately, but certainly over time. This isn't mold but cocoa butter, a telltale sign of chocolate that's come out of temper. Even if your dipped desserts look perfectly glossy, it's normal for this to happen to the dregs of chocolate scraped up from around the sides of the bowl. Again, not great for aesthetics, but easily hidden in many desserts.
You can peel up the hardened chocolate to stash in a zipper-lock bag, where it's easily broken up, but I like to chop it into bite-size pieces for ease of use. Due to potential contaminants and the likelihood that your chocolate is no longer in perfect temper, it's best to store that bag o' leftovers in the fridge. Refrigerated, the chocolate can be kept for a few months.
Whether it makes up all or just a portion of the chocolate you need, leftovers work well in any recipe in which the chocolate will eventually be subjected to some heat, like baked goods or stovetop custards. If you like, pudding and ice cream bases can be strained to remove any cookie crumbs buried in the chocolate, but as often as not, I consider such hitchhikers a bonus. Graham cracker crumb chocolate ice cream? Yes, please.
So mix it into a bowlful of malted cookie dough, melt it into the silky custard of a chocolate cream pie, or toss it into a batch of bakery-style chocolate scones, but whatever you do, don't let that leftover chocolate go to waste.