Is the Buddha nature nothingness or the Dharmakaya?
According to Yogacara, buddha nature is a seed in the mind basis of all. According to Madhyamika, buddha nature is the emptiness of the mind of a sentient being. It is not the dharmakaya.

Dharmakaya is divided into two (sometimes three): 1. the wisdom truth body, which is the omniscient mind of a buddha, and 2. the nature truth body, which is the emptiness of the mind of a buddha. We also call the nature truth body: 'emptiness [of the mind] without defilements' while buddha nature  is 'the emptiness [of a mind with] defilements'. Just like the emptiness of the table is not the same phenomena as the emptiness of the cup, the emptiness of the mind of a sentient being (i.e. a non-buddha) is not the same as the emptiness of the omniscient mind of a buddha. Therefore, we can not say that buddha nature is the dharmakaya or that an arya buddha possesses buddha nature.

Moreover, in no traditional text have I seen the dharmakaya qualified as 'transcendent'.