Buttercream and fondant are both used to decorate cakes, but buttercream is a whipped, fluffy frosting that can be spread and piped, while fondant is a smooth, pliable icing that can be rolled out and sculpted. Buttercream is primarily made of butter and sugar, and fondant is primarily made of powdered sugar, water, and glycerin or marshmallows.
Buttercream frosting is similar to fondant in the way that it can last at room temperature for three to four days, as long as it’s protected from dust with a cake box or dome.