Cookies are one of the most loved and readily available baked goodies, which are easy to assemble and barely take much time to bake. But, when you add something that does not readily hold shape, like chunks of chopped or snapped chocolate pieces from a good quality chocolate bar, it's important when and where you add them. This holds for the likes of cookies like pistachio shortbread cookies with Dairy Milk chunks, and also other varieties of cookies where the chocolate chunks will melt into gooey pools of rich chocolate and enhance the whole experience of enjoying the cookie.
This is one area you should not skimp on if you want quality output. To attain this, always go for high-quality chocolate bars, not chocolate chips or even cooking chocolate. While chocolate chips are designed to hold their shape, so they won’t melt as easily, a chocolate bar, on the other hand, compensates in taste. So, chop up a good-quality chocolate bar, like Cadbury Dairy Milk chocolate, Silk, or Bournville, and distribute the irregular chocolate chunks into the cookie dough. You could also press some chunks on the surface for an extra chocolate pop; these will melt well into the dough and make for a memorable batch of cookies, whether or not other flavors like nuts and raisins are in the cookies.
So, when is the best time to add the chocolate chunks, given their rough shape and fragility? If you are chilling your dough, wait to add the chocolate chunks until right before baking, or just press a few chunks of the chocolate pieces on top of each cookie dough ball just before they go in the oven. This will ensure visible, at least the latter, you end up with melty pools of chocolate. If you're scared of making a mess, push the chocolate chunks into the individual portions of cookie dough on the baking sheet to yield calderas of molten chocolate post-baking.
The aim is a molten pool of chocolate distributed in the cookies, which aren't too runny and stay in place, but when you bite into a warm gooey you feel the softness of the almost melted chocolate chunks. So, to achieve this, you need to slightly underbake the batch of cookies. Stop around a minute before the cookies are fully done, check for golden edges and a softer center; your cookies will still be baking once you remove them from the oven, because of their lesser density, size, and shape, so keep that in mind. This step will help retain that soft, gooey texture of the chocolate in the cookies and also keep the chocolate from solidifying into shards.
There's no fixed recipe to make cookies, so you can combine different kinds of chocolate, for example, a mix of dark and milk chocolate chunks or even white and dark chocolate chunks, or go for all 3 if you have all at hand for a contrast of colors and also flavors. The 3 different chocolates will have different melting points, courtesy of the differing cocoa content (zero for white chocolate), so keep that in mind and add them accordingly. Their almost gooey versions will also taste different. This idea is excellent for bland cookies, which are just plain flour, butter, and sugar, and nothing else, to bring out the best of the trio or duo of flavors of the chocolate chunks added to the cookie dough.
Like brownies, which are a lazy person’s dessert for a reason, cookies also benefit from reheating to activate the solidified chocolate chunks in them. You can, of course, enjoy your just-baked melty cookies fresh, while they are still warm, but in case you are baking up a storm, reheat whichever amount you're having briefly in the oven or microwave, for 5-10 seconds, to re-soften the chocolate without overcooking the cookie. This brief period of heating also allows the cookie’s innate flavor to activate along with the chocolate chunks going molten again, and after all, a cookie always tastes best warm, as most bakers would agree.