Meanwhile, make the buttercream. To make the caramel, dissolve the sugar and 60ml water in a small, solid-based pan over a gentle heat, then increase the heat to a boil. Wait a few minutes, leaving the pan undisturbed but watching it like a hawk and, as soon as it changes to a wonderful caramel colour (like strong tea) and is thicker, remove immediately from the heat, stand well back, and add the cream. Be very careful as it is searing hot and it may splatter a little. It will react, or ‘seize’, and you may think it has gone wrong; it hasn’t. Keep stirring, adding the salt and the vanilla. Leave until cool. If using dulce de leche, simply mix it with the salt and vanilla.
Cream the butter and icing sugar for at least 5 minutes in an electric mixer (or use a handheld mixer) and add the caramel or dulce de leche. Put the buttercream into the piping bag fitted with a star nozzle and pipe it on to the cakes, or spread it on with a palette knife.