"Unfrosted" refers to food or objects that do not have a layer of frosting or icing on them. It means they are plain or without the sweet coating often found on cakes, cookies, or other baked goods.
Freeze unfrosted cupcakes in freezer weight resealable plastic bags for up to 3 months. (snackworks.com)
Brownies would work well or a different type of unfrosted cake (icing would catch on the skewer so I would skip anything with icing!). (staceymattinson.com)
Or about how much I think frosted brownies knock the socks off of unfrosted ones because anything more closely resembling cake is better than anything less closely resembling cake. (feastingonfruit.com)