Cake:

Preheat oven to 350 degrees; butter a 9-inch spring form pan.

In a large bowl (preferably metal), whisk the egg whites and salt until stiff. In another bowl, cream the butter and Nutella, then add the rum, egg yolks, and ground hazelnuts. Fold in melted chocolate.

Add a blob of beaten egg whites to the chocolate batter, and mix gently until well-combined. Fold in the remaining whites, one-third at a time, very gently but thoroughly. Pour into spring form and bake for 40 minutes. Test for doneness by inserting tester, which should come out mostly clean; lightly pressing finger into top to check for a slight bouncing-back; and observing edges beginning to separate from pan. Let cool completely, in pan, on a rack.

Frosting:

Place the butter and peanut butter into a medium bowl, and beat with an electric mixer. Gradually mix in the sugar, and when it starts to get thick, incorporate milk one tablespoon at a time until all of the sugar is mixed in and the frosting is thick and spreadable. Beat for at least 3 minutes for it to get good and fluffy.