The key is a crispy cookie, but a cookie that does not crumble when being cut with a knife.

Test a cookie by cutting the cookie gently. Does the cookie crumble?

If no, you are good to proceed.

If yes, microwave the cookie(s) in increments of 5 seconds to ever so slightly soften the cookie.

I begin with microwaving 10 seconds on high and then start the 5 seconds increments.

Test the cookie again by doing a test cut with the knife. If the cookie can be cut without the cookie crumbling you are good to begin the heart shaping.