layers are usually the warmest way to dress instead of just wanting a warm jacket, but for a specific item to be warm you want it trapping air like a down jacket, but they can suffer if they get wet. So, a down gillet under a decent windproof/waterproof is probably the best way. I'm just thinking of my snow gear when it's brass monkeys (which I've been down to colder than -30), it's been thermals, then longsleeve t-shirt, then fleece or gillet, then jacket with snood thing. 1 of my jackets has a metallic weave inside which reflects/traps body heat and helps, but another is like a puffa thing that just relies on trapping air. Need to build up layers to control temp.
