Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.2.13

2.5.2.13

atha kāya ekakapālaḥ puroḍāśo bhavati | kaṃ vai prajāpatiḥ prajābhyaḥ kāyenaikakapālena puroḍāśenākuruta kamvevaiṣa etatprajābhyaḥ kāyenaikakapālena puroḍāśena kurute tasmātkāya ekakapālaḥ puroḍāśo bhavati

Then follows a cake on one potsherd for Ka (Pragâpati); for by that cake on one potsherd to Ka Pragâpati indeed bestowed happiness (ka) on the creatures, and so does he (the sacrificer) now bestow happiness on the creatures by that one-cup cake: this is why there is a cake on one potsherd for Ka.