@georgia here's what it says:
Both, at different levels of discourse — but in Advaita's final reckoning it is a kośa and the kāraṇa śarīra, not Brahman.
The adhikaraṇa. This is not a settled point across schools; it is the subject of Brahma Sūtra 1.1.12–19, the ānandamayādhikaraṇa, and the ācāryas split.
Śaṅkara concludes that ānandamaya is a sheath, and that Brahman is what the śruti indicates by brahma pucchaṃ pratiṣṭhā — the ground, not the ānandamaya itself. His grounds: the -maya suffix denotes vikāra; the ānandamaya is described in Taittirīya 2.5 as having avayavas (priya as head, moda and pramoda as wings); and a partite, modified thing cannot be niravayava Brahman.
Rāmānuja rejects this in the Śrī Bhāṣya. He reads -maya as prācurya — abundance — so that ānandamaya means "consisting in fullness of bliss," i.e. Brahman itself. It should be said that sūtra 1.1.13, vikāraśabdān neti cen na prācuryāt, reads more naturally in his favour, and Śaṅkara's handling of the adhikaraṇa is textually strained. Vācaspati defends him in the Bhāmatī. But the later Advaita manuals — Vedāntasāra, Pañcadaśī 3 — proceed uniformly on Śaṅkara's conclusion, and that is the frame you are presumably working in.
Why it is a kośa. Its bliss is graded: priya, moda, pramoda are degrees of intensity. Brahman admits no degrees. Gradation implies conditioning, hence an upādhi. What is present is ānanda-pratibimba — bliss reflected in sattva-predominant mūlājñāna — not svarūpa-ānanda. It also arises and subsides: it is what obtains in suṣupti and not otherwise. And it is objectified — "I slept happily, I knew nothing" is a recollection, so something witnessed it. The sākṣin is not the ānandamaya.
The concealment is subtler than the other four, and correspondingly more dangerous: this sheath does not hide bliss, it substitutes a very clear reflection for the original.
Why the śruti calls it ātman. Taittirīya 2 uses ātman serially — each kośa is said to be the śārīra ātman of the one preceding. This is progressive interiorization, a pedagogical device of relative reference, not a series of ontological claims. Ānandamaya is ātman with respect to vijñānamaya. The paramārtha ātman is what survives the neti-neti of all five.
Māṇḍūkya settles it. Prājña, the causal state, is ānandabhuk, the enjoyer of bliss — and prājña is the third pāda, not the fourth. Turīya is described negatively at MU 7: not inwardly cognitive, not outwardly cognitive, prapañcopaśama, śānta, śiva, advaita. "Enjoyer of bliss" retains bhoktṛtva, hence duality. Ānandamaya maps to prājña; Brahman is turīya. The bliss of the ānandamaya is still eaten.
On the metaphor. As with the storage image previously: kośa is not a literal stratification. The five are not concentric shells around a kernel. They are the single reflected consciousness described under progressively subtler upādhis, and the "innermost" is not spatially deepest — it is what the whole series is an appearance of. Ānandamaya is the last description before description fails; that proximity is exactly why it is mistaken for the thing itself.