This vanilla frosting is very stable, so you can ice the cake before you transport it. The frosting will be soft in warm weather and firmer in cool weather. The cake is best eaten on the day of making.
Note For a whiter frosting, use a European-style butter with a pale colour. It's important the meringue is completely cooled before the butter is added; otherwise, the butter will melt. If the mixture is too cold, it may look curdled; if this happens, continue beating the mixture until it is smooth.