Deriving hex private key from a HD wallet I have the following test Mnemonic:

stable primary already collect buffalo bike into robust fury cement proof agree neither blouse course


From this I can generate a BIP39 Wallet:
https://iancoleman.io/bip39/

From this I can get all the private keys of the derived addresses (if you scroll down to the bottom).

But I'm not sure what format they're in. 

The library, bitconjs-lib does the following:

const ONE = Buffer.from('0000000000000000000000000000000000000000000000000000000000000001', 'hex')
keyPair = ECPair.fromPrivateKey(ONE)


https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/test/ecpair.js#L29

But the private keys generated by https://iancoleman.io/bip39/ do not match the length required by bitcoinjs-lib
The mnemonic represents a BIP32 extended private key: xprv9s21ZrQH143K4BbpSvRBDykoXLhwtznbp7qAfKVLw1QdJtJsmgS5rZJfSgZNtgk7e39uRNiYHbcRQnYvruqfftfG6WFmuiCspehEz1VS3YM

The function you are calling accepts a regular private key.

You can derive child private keys from the extended private key and use these to generate addresses.

It it be possible to derive the corresponding private key of either the children of this extended private key or the corresponding private key itself using the toWIF method in the bip32 JS library. See https://github.com/bitcoinjs/bip32/blob/master/index.js

You can see some examples of the use of BIP32 here:
https://github.com/bitcoinjs/bitcoinjs-lib/blob/a6f2cc2924df170537f0b588254178f5445d7dc8/test/integration/bip32.js