Shatapatha Brahmana 6.6.1.9

6.6.1.9

athādhyātmam | śira eva vaiśvānara ātmaiṣa ādityaścaruḥ śiraśca tadātmānaṃ ca karoti vaiśvānaram pūrvaṃ nirvapati śirastatkṛtvātmānaṃ karoti

Now as regards the self (or body of Agni). The Vaisvânara (cake) is the head, and that Âditya pap is the body: he thus makes both the head and the body. The Vaisvânara (cake) he prepares first; and having thereby made the head, he then makes the body.