What makes a cooky chewy?

Boulangere
It worked. Thank you! I wanted a chewy chocolate chip cookie.

Add about 2 ounces of corn syrup to your dough at the stage when your are creaming the butter and sugar(s). It will prevent the sugar(s) from recrystallizing after baking, making your cookies softer and more chewy.

I've always found cookies made with margarine to be flatter and crispier than those baked with butter which tend to be softer and chewier.

Usually the combination of a flour with gluten (AP or bread flour) and melted butter. These two ingredients combined in the dough, makes a cookie very chewy.