Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.2.10

1.7.2.10

sa ājyasyopastīrya | dvirhaviṣo 'vadāyāthopariṣṭādājyasyābhighārayati dve vā āhutī somāhutirevānyājyāhutiranyā tata eṣā kevalī yatsomāhutirathaiṣājyāhutiryaddhaviryajño yatpaśustadājyamevaitatkaroti tasmādubhayata ājyam bhavatyetadvai juṣṭaṃ devānāṃ yadājyaṃ tajjuṣṭamevaitaddevebhyaḥ karoti tasmādubhayata ājyam bhavati

Having made an under-layer of butter (in the guhû-spoon) and cut off twice from the havis, he then pours over it some butter. There are, indeed, two (kinds of) oblations; the oblation of Soma being one, and the oblation of (or rather, with) butter being the other. Now the one, viz. the Soma-oblation, is (an oblation) by itself; and the other, viz. the butter-oblation, is the same as the offering of havis (rice, milk, &c.) and the animal offering; hence he thereby makes it (the cake) butter, and therefore butter is on both sides of it. Butter, doubtless, is palatable to the gods; hence he thereby renders it palatable to the gods: for this reason butter is on both sides of it.