Yes! Traditional swiss meringue buttercream calls for caster sugar instead of icing sugar. To use caster sugar in this recipe, you will need to dissolve the sugar into the egg whites before using your electric mixer. Place the egg white powder, water and sugar in the bowl of your stand mixer. Place this over a pot of simmering water (as a makeshift double boiler) and whisk until the sugar dissolves. As the egg whites are already pasteurized, you do not need to heat the mixture above a certain temperature to pasteurize the eggs again. Then continue from step 2 of this recipe. If your feeling particularly lazy, I have heard of bakers heating the egg whites/water/sugar in the microwave instead of using a double boiler - although I haven't tested this method myself.