I usually just do a direct substitution- but it is noticeable in the end. It is just as moist, tastes a little different, and softer. I think it works better with oatmeal cookies because the apple flavor goes better with oats and ginger etc... than chocolate chips. You could cut half the butter and use the same amount of applesauce to replace the rest of the butter (1 stick=1/2 cup applesauce)
What exactly are your goals in the substition? If it's to cut fat, then you can also drain some vanilla yogurt (use cheesecloth) and use that instead; I've also experimented with ricotta cheese with reasonable success. I've also used a combo applesauce and pureed pecans (creates a thick dust) and the pecans make them buttery, but the texture is softer and not as crispy. Sorry for the rambling, hope this helps!