How long for buttercream to go hard?
The time required for buttercream to develop a firm crust
depends on a number of factors, including the proportion of
ingredients used, the temperature of the ingredients when combined,
and the temperature and humidity of the room. Generally, if you
want to be certain the buttercream crusts over fully, you should
leave the cake uncovered overnight. It will crust more quickly in
the refrigerator, but will still take several hours.
> It would take several days for buttercream frosting to
become completely hard all the way through. Even then it will be
quite soft. Hard icing decorations are not made out of buttercream
but out of royal icing.