A Chestnut Mushroom is the same mushroom as White Button Mushrooms, but it is a strain that grows just a bit browner instead of white, giving it a tan-coloured top. They are very much like Cremini Mushrooms and about the same size. They have better flavour and texture than the plain white mushrooms. Chestnut Mushrooms are also allowed to grow into larger sizes to be sold as ?Portobellos.? Sometimes Portobellos are also called ?Flat Chestnut Mushrooms? for this reason. Chestnut Mushrooms are a member of the Agaricus family of mushrooms.