The sous chef's toque is an item obtainable from the Squeal of Fortune. When worn, it provides a 1% boost (up to 5% when worn with the rest of the Sous chef's outfit) in experience when training cooking.
When wearing this hat while trying to enter the cooks' guild you aren't allowed to enter, even while the Head Chef tells you should wear a chef's hat or something like that.
On the first day the Sous chef's outfit came out, All the items were briefly under the Uncommon slot. They were moved to the Rare slot shortly after.