In a large bowl, rub the melted butter with the sugar, salt and vanilla.
Add the cocoa, gradually beat the eggs lightly and mix.
Mix the baking powder with the flour. Gradually integrate the flour and finally the chopped hazelnuts.
Pour into a greased dish.
Bake at 180 ° C for 20 minutes or until the toothpick inserted in the center comes out almost clean.
Remove from the oven, and cover evenly with Marshmallow.
Bake again for 5 minutes.
Melt the chocolate in a "bain-marie" with 1 tablespoon of milk (or water).
Using a spatula, stir vigorously until glossy.
Sprinkle the hot cake with chocolate icing.
Allow to cool until the chocolate hardens.
Cut portions and serve.