The crinkle effect is due to these double chocolate crinkle cookies being rolled in powdered sugar and then the cookies spreading a bit in the oven before the edges set. If the cookie edges set before the cookies can spread, you won't get the desired crinkle look. A few reasons this might happen is your oven temperature is too high (use an oven thermometer), you are baking on too high an oven rack, or have too much flour in your cookie dough.