+1.
My helmet is very snug (fnar) so a thick cap won't work for me, but I can use a Buff.
My face and ears get cold, so I pull it on normally to gather around the neck area, draw up the top 15cm or so to cover the lower face and ears, plonk helmet on head, pushing down any spare material to just under the helmet rim, then fasten chinstrap in place over the Buff. This also keeps your jaw and mouth reasonably warm.
If you're someone whose head primarily gets cold, you can form your Buff into a skullcap. It's very thin material so it'll fit inside most people's helmets.
If you get really cold all over your head, you can buy two, and do both!