def METHOD_NAME(plaintext_data_key):
    """
    _plaintext_data_key returns the 'Plaintext' value from the response.
    It caches the response and only calls _api_decrypt exactly once.
    """
    with patch.object(
        aws_kms,
        "_api_decrypt",
        return_value={"KeyId": "key-id", "Plaintext": plaintext_data_key},
    ) as api_decrypt:
        assert aws_kms._plaintext_data_key() == plaintext_data_key
        aws_kms._plaintext_data_key()
        api_decrypt.assert_called_once()