My serious cold weather gloves are a pair of Columbia skiing gloves that I bought many years ago when I used to travel regularly to the Nordic areas in winter. No problem at all in sub-zero, and my hands will warm up in them if they've been outside and got really cold for eating or fixing something on the bike. A bit bulky, but fine on STIs after a bit of getting used to them, and waterproof too.
I hadn't realised that muffs were a thing on bicycles; I've used them on the motorbike to great effect. Summer gloves are fine well below zero in that case, and the added benefit that mine cover the troublesome glove/jacket interface and stop water working its way in during torrential rain. I'll have to investigate the bicycle version, though at the moment I'm doing OK in a pair of winter sealskinz gloves.