You have a couple of options - whichever you choose if ordering specifics you need to give at least 24 hours unless just picking something out of display case at bakery:
#1 - Contact CRT directly (call disney dining and ask for #) and make arrangements with the restaurant. This will probably be the easiest as you won't have to do anything other than show up for reservation and pay extra for the cake - usually around $25-$40 depending on what you choose. They don't take orders closer then 2 weeks and need at least 48 hours for order. They may ask for yor PS number also so make sure you have it handy.
#2 - You could also go to the Boardwalk Bakery and let her pick out her own cake to bring to your reservation with you. You could also pick it up on your own vs. having her pick it out of the display case.
Disney Dining explained to me that you can order specialty cakes...birthday, wedding, anniversaries, etc. at any resort or resstaurant. You need to contact the restaurants directly and work with them. If you want it shipped to your resort/room (I want 1 in our room at All Star Sports for our anniversay in October) I was told to contact the food court manager/personnel directly in our hotel and they will work with us. I was assured that yes indeed it could be handled for us w/o concierge/being at the GF/or picking up the cake myself and hope that it will indeed happen for our event. I do plan on printing out a picture I found on the disboards that I like in case I get there and they look at me puzzled? I will also post what happened when we get back...