UKH

As per title, can anyone recommend a thick, knitted beanie for extreme cold weather, I'm thinking -15 degrees C.
Ta

ME flash beanie has been keeping me warm in temps down to -20 (and further with a balaclava). Fits nicely under a helmet too.

I quite like a wool hat with fleecy lining. I have a very cosy one which is tweed with fleece lining. I find the combination of the fabrics works well. 

I buy mine from wee local crafty places or make them myself, so can't actually recommend a particular one. 

I haven't found a hat better than the Fjallraven classic knit to keep my slaphead warm in properly cold weather. 100% merino wool, super warm, breathes well, lasts forever, doesn't bobble.

I would recommend you look at blackyak.co.uk for a great range, good service. 

Knitted wool with an internal fleece band.

The best hat for extreme cold is a hood, as you get full ear and neck coverage. In even more extreme cold you can't beat a 'ushanka' style hat, the ear warmth is unmatched and on a good one you can tie your ear flaps up for beanie mode

The possum fur and merino beanie from the Ray Mears shop is by far the warmest wool hat I've worn. Somebody got me one for a present a few years ago but I've only worn it a few times since, because it's too hot on all but the bitterest days. It also looks like the sort of hat Ray Mears would use.

I have several wool beanie hats.  The warmest is a 3M Thinsulate . it's also supposed to be waterproof, but I have no plans to ever test that theory.  I bought mine at the local mill shop a few years back.

https://www.amazon.co.uk/Thinsulate-Waterproof-Beanie-Windproof-Thermal/dp/...

My oldest kid got me a possum/merino cardie for my birthday - an amazing fabric. Much better than my tattered goose down. I thought it sounded a bit 'gimmicky' when I got it, but I'm sat in the 4am garden at -5 and perfectly toasty. 

Aclima WarmWool Beanie has thick wool and a clever fish net style lining, this and the Brynje equivalent are very warm and popular in cold if you want a beanie rather than mountain cap / ear flap style hat. They are expensive though.

Any wool beanie will do just fine, as long as it isn’t those thin under helmet beanies.

thin fleece liner makes them less itchy.

but If it is windy, you’ll need something windblocking to go ontop of it. A hardshell hood works fine.

+1 for the fjallraven. If you don't need it under a helmet, it's great. I've just bought my second 

I use a super one from 'Nepal' , that's the brand; but hand knitted by ladies in the country

My hat knitted by the Sherpas, with good cover for my lugs is really toasty on the morning dogwalks.

I've a Fjallraven Expedition down hat thingy, it's actually hideous when it's only -10 or so as one sweats like hell, it was a gift and I never use it!

To be fair, it does say that hat is designed for dog sledding! The classic knit wool cap breathes nicely. 

It's fairly thick, but not so thick you can't shove a helmet over it when it's baltic.