Butter cream is a cake filling and coating made by whisking butter with an approximately equal weight of icing sugar or caster sugar until light and fluffy. The beating incorporates air while the sugar dissolves into the fat, producing a smooth, sweet, pipeable frosting used on cupcakes, layer cakes, and celebration cakes. It can be flavoured with vanilla, chocolate, coffee, or fruit purées, and lightened by replacing some butter with milk or cream. Because it is high in fat and sugar, buttercream is energy-dense and best enjoyed in moderation. It must be kept cool in warm weather, as it softens and weeps when overheated.