Hey, that sounds like fun. I'd start with these flavorings by The Flavor Apprentice: Molasses, Pie Crust, and Brown Sugar Extra. The molasses is a mild flavor, believe it or not, and really true to the real thing. The brown sugar is a very strong flavoring, so start low there. Pie Crust is great with other "fillings" - never tried anything like you're doing. Absolutely no guarantee it will be right, but I'd start with 2% Pie Crust, 2% Molasses, and just 1% Brown Sugar. Adjust those up/down until the balance is about right, then figure out if your grandma included cinnamon or other spices that you want to add. Good luck! I hope you'll tell us how your experiments go!