I've recently picked up a couple nice pieces of clothing that I can recommend for layering in cold weather.
First, is a French quarter-zip fleece top in CCE camo. I got it from Varusteleka but it's available at many surplus suppliers. As with any military surplus clothing, size choices are limited. It's a very simple light fleece shirt but it's surprisingly warm especially for its weight, which is only about 270 grams. If you can find one in your size grab it while they are available.
Second, is a Rothco copy of the old USGI 5-button or "Jeep" sweater (eBay link). I've had the real deal in the past but I put on weight so I gave them away. Genuine GI sweaters are still available but mostly in small sizes. They were made in wool knit or acrylic knit variants. Wool is warmer but the acrylic sweaters are still pretty warm. (The above is an eBay link. Here's an Amazon affiliate link.)
Note that by design these are snug-fitting and were often worn over a base layer but underneath a shirt, especially the old M-1951 wool field shirt or a BDU shirt.
Third is a Sarma TST L1 merino wool balaclava, also from Varusteleka. This appears to be made from the same material as their L1 merino wool t-shirt. In other words, it's really thin and packs up very small, to about the size of a pack of cigarettes. I got the L/XL size. For reference my hat size is 7-3/8 or 58/58cm.
Yesterday I took a half hour walk in 20*F weather. On top I wore a long sleeve Hill People Gear synthetic t-shirt, the Rothco sweater, my Arc'Teryx Atom SV jacket, topped with a German surplus flecktarn parka to provide a windproof outer layer that covered the top half of my thighs, and the Sarma balaclava. About halfway through the walk I started overheating and had to drop the hood and unzip a little.
I was very impressed by the Sarma balaclava. Despite its thickness (or lack thereof) it made a noticeable difference. It can also be rolled down for use as a neck tube/gaiter, or rolled up for use as a beanie. It's stretchy enough to pull down and expose your mouth. I'll be interested to see how it does in warmer weather.