It’s important to keep those ears toasty in winter. I use homemade earflaps.
Here is a picture of my daughter and I with earflaps. (photo credit Graham Paine)
I find that they are better than a hat under the helmet for moderately cold or windy conditions since I have the option of covered ears without blocking the helmet vents. They can always be combined with a hat or helmet cover.
In these images, you can see that they are a simple piece of cloth (in this case, red fleece) with strips of velcro. I got these earflaps from a member of my former bike club (TCBA, in Lansing MI; a really great bike club BTW).
I’ve made up a pattern if you want to make your own: earflap pattern
If you can’t sew, or don’t have access to a machine, you can always use staples to attach them to the helmet. I’ve had mixed luck with self adhesive velcro.
Make some up, and keep those ears warm!