When you chop up a bar of chocolate, you end up with large Chunks and tiny little flakes of chocolate. The larger chunks look beautiful and create these delicious-looking puddles of chocolate. The tiny bits get fully incorporated into the dough, making a speckled look, and distributing the chocolate flavor. Chocolate chips are designed to hold their shape and texture at much higher temperatures, so you end up with concentrated little pockets of chocolate. You can obviously use whatever you have, but I love how the chocolate chunks look in these cookies. It makes it look like a fancy cookie from a bakery rather than something I got in the package.