Anandakanda 1.21.54

1.21.54

tadbahirbhūpuraṃ lekhyam enadāghorayantrakam /
sphuradvayāvṛtaṃ madhye śaktibījaṃ likhettataḥ //

Outside that, a bhūpura should be drawn: this is the Aghora yantra. / Surrounded by the two “sphura”s, one should then write the Śakti seed in the center;