Heat the maple syrup and keep warm. Beat the eggs with the water until doubled in volume, then slowly drizzle in the still warm maple syrup, turning the speed to high if you're using a stand mixer.
When all the maple syrup is added, add the butter a couple pieces at a time, until it is all incorporated and the mixture is smooth.
Finally, add the salt, then transfer to a container. Keep the buttercream at room temperature until you're ready to frost the cake. Butter cream can be frozen for months and then allowed to thaw at room temperature and whipped to fluff it up lightly before using.