Hi Mo, they shouldn't be stored for more than a day or they'll lose their soft centers. If you want
to do them in advance, I think the best way would be to make the dough, shape the cookies, freeze them, then bake the night before serving, and store in an airtight glass canister. The most important thing is NOT to overbake - if anything, underbake them, ie, until they still feel raw to the touch in the centers, as I've noted on the recipe.