For the pastry cream, whisk the egg yolks, sugar, salt & cornstarch. Simmer the milk & butter, then stream the milk over the egg mixture while whisking.
For the buttercream, whisk egg whites and sugar over a double boiler. Beat until the meringue is at room temperature & then add butter, vanilla bean paste and salt.
Sprinkle tops of cake with sugar and torch it to caramelize. Pipe a border of buttercream on the first two layers & fill with pastry cream. Refrigerate the cake.