re: Buttercream help please

Thanks June. I usually use butter with double quantity icing sugar to go under fondant/sugar paste.
SMBC (heated egg whites) caster sugar and butter for cupcakes. Didn’t know if was suitable for covering a whole cake? Is it ok left at room temperature or will it be too soft? The cake needs to travel 250 miles so want a recipe that will be stable and still look good!!

Sarah x