First, I have my sites at BlueHost and I do not know anything about how GoDaddy deals with Add-On Domains. At BlueHost, I simply cPanel-assign a given domain to its installation folder and that is the end of that. So, I have this:
primary.com assigned to /public_html/
add-on1.com assigned to /public_html/addon1/
add-on2.com assigned to /public_html/addon2/
add-on3.com assigned to /public_html/addon3/
add-on4.com assigned to /public_html/addon4/
add-on5.com assigned to /public_html/addon5/
'iambiscuit.com' is a registered domain that never needs to know it is an Add-On. So unless your account at GoDaddy does not allow for more than one domain, you can do away with any tricky stuff by simply pointing 'iambiscuit.com' to the GoDaddy nameservers and then just cPanel-assign 'iambiscuit.com' to its own folder and never have anything other than 'http://iambiscuit.com' at Settings > General and/or anywhere in your database.