Although this wedding cake is covered with a fondant icing, the buttercream layer underneath is essential both for flavour and to create flat sides and tops to the cake layers.
Whisk the sugar and egg whites together in a metal bowl. Place the bowl over a pot of simmering water and whisk by hand until the sugar has dissolved and the mixture is hot, about 3 minutes. Remove from the heat.
Switch to electric beaters (or a stand mixer) and whip the whites until they have cooled to room temperature (they will more than double in volume), a