Combine the egg whites, sugar, salt and vanilla in a heatproof bowl and place over a hot water bath.
Keep over heat, stirring all the time, until the mixture is hot and the sugar has dissolved completely.
Remove from heat and start mixing with an electric mixer until you get a meringue that has a shiny look and it's stiff, while the bowl has gone completely cold.
Add the butter, all at once, and mix well on low heat for 2-3 minutes until the mixture curdles then comes back together.
When done, split the buttercream in 2/3 and 1/3.
Mix 2/3 of the cream with dark chocolate and the 1/3 with white chocolate.
Spoon both buttercreams in two piping bags fitted with a round tip.