I wanted to make some NFSC with MMF and royal icing accents for a baby shower but now realize they need to be kosher. Forgive my stupidity, what constitutes a kosher cookie?
That's a complicated question... a truly kosher cookie would be one that uses only ingredients that are certified kosher, and is made in a kosher kitchen.
However, if the person who you are making the cookies for isn't *strictly* kosher, you might be able to use kosher ingredients and bake in your regular kitchen. Or, even use ingredients that aren't specifically certified kosher, but follow the general rules (ex. not mixing dairy and meat).
You'll really need to ask the person who the cookies are being made for what level of "kosherness" is needed. There isn't anything in the NFSC recipe that would be against the kosher "rules," but it will depend on the level of strictness as to what brand of products you can use and whether or not it can be made in the kitchen you usually use.
Oh - MMF might pose a big problem for you, as most marshmallows are made from gelatin, which is typically animal derived and would NOT be kosher.
A member here, kathik I believe specializes in Kosher baking. You might try asking her.
Bonnie gave you a good synopsis of making kosher cookies. I make NFSC all the time. Since I do only non-dairy baking, I use margarine instead of butter.
Depending on where you live, there are kosher marshmallows, and I have used them to make MMF. I found the fondant dry, but was able to work with it.
Take note that Kosher ingredients are more expensive you'll need to charge more.
Kosher marshmallows are made from fish gelatin instead of animal gelatin.