I think this is a great idea. New way to celebrate birthday parties and you can even mail these if you think it will make it in 3 days or less in order to keep it fresh.
Next time you give a birthday party think of making a cake in a jar.
One of my best memories of growing up was when I was 16 a family friend made me a birthday cake in a coffee can. I still remember that. Being born on Christmas Day there were not but maybe one or two birthday cakes I can remember. I will never forget the one baked in a coffee can. So next time you need to make someone a cake, make it in a jar and serve it up with some pretty colorful forks and spoons. You could really get creative and get the glass jars that have the handles and let the guest take them home. I wonder how it would be to make the birthday cake then layer it with ice cream like those at Baskin Robbins. You could put a layer of cake, layer of Ice cream then layer of cake and icing. Oh the ideas are endless!