Butter and sugar make the foundation of the cookie base, so they must be creamy and homogenous.
Chilling makes the dough firm and locks in the moisture so it won’t dry out while baking.
Brown sugar infuses more moisture than white sugar due to its high molasses content.
Bake the cookies only until they are just set to keep the center slightly gooey.
Press the chocolate in the center at the right time after taking the cookies out to ensure the chocolate melts only slightly.