Actually as OP asked for a pem format (following from example above):
$ openssl ec -inform d < btc_priv.key
read EC key
writing EC key
-----BEGIN EC PRIVATE KEY-----
MC4CAQEEIC1yOBBt+H17cPnXzE/4M/+frYaqXlD4z0JonCONcPcjoAcGBSuBBAAK
-----END EC PRIVATE KEY-----