Mix the flour, salt, egg yolk and granulated sugar together with just enough cold milk to make a pliable mixture. Beat the egg white together with the granulated sugar until stiff and lift under the mixture.
Heat some butter in a pan, add the mixture, as well as the raisins that have been soaking in rum, and fry the mixture on both sides until golden brown. Use two forks to tear the mixture into smaller pieces, sprinkle these with Wiener icing sugar and leave on the stove until they are fully baked and slightly caramelised.
Spread onto preheated plates, generously sprinkle with caster sugar and serve with a plum compote.
Plum compote (Zwetschkenröster) is typically Austrian and difficult to describe accurately, as it is neither a real compote nor a typical mousse; it is in fact a mix of the two. This mix can be served as a compote on its own, or as an indispensible side dish.