Woolen mittens handmade with the naalbinding knitting technique. Naalbinding was the knitting technique used during the Viking Age and it has survived in the Nordic Countries till this day. If you want historically accurate Viking Age mittens then these are the ones to go with. They do an excellent job with both ventilating and keeping your hands warm.
Clothing remains from the Viking Age are rare due to wool, linen and such not surviving very well. There are, however, two known naalbinding mittens that have survived. One from Iceland and one from Olso in Norway.