Is it sugar, leavening agents or amount of flour used that will promote the crack on top of cookies?
The crackled texture on top of cookies is more common on softer, moister, cake like cookies such as chocolate crinkles or ginger bread cookies. Here, in this image, it is accentuated because the raw dough was rolled in powdered sugar:
This affect is achieved by balancing the baking temperature and amount of moisture in the recipe so that the outside of the cookie will set firmly enough that it cannot expand before the inside of the cookie heats through.
When the inside of the cookie heats gets hot enough to activate its leavening and it expands, the outer shell cannot expand with it, and so it cracks. The fissures are from where the inner cookie shows that the broken shell of the inner surface.
A second, different phenomenon occurs in other types of cookies. In one of your other questions, you linked to a recipe with this image:
This is achieved by allowing the cookie to over-rise, and then collapse back upon itself. There are two ways to do this:
Over-leaven the cookie with baking powder. They will rise up and then collapse in the oven. This requires a moderate baking temperature so that they don't set before collapsing.
Under-bake the cookies, so that they are not fully set when they come out of the oven, and then collapse under their own weight as they cool.
I find that using these tips help me achieve the crackled texture on my cookies.
Chill your dough for AT LEAST one hour before putting in the oven.
Make sure you don't use old raising agent. Sitting in the cupboard for too long may make it lose its properties.
Roll them into balls and DON'T press them down before putting the in the oven. If you chilled them before, the should spread out and give you that crackled texture.